|
|
Assessment of Iodine
Deficiency Disorders and Monitoring their Elimination : A guide for programme
managers, Second Edition. ICCIDD/UNCF/ WHO, 2001
Continued . . .
School-based surveys
If a
school-based survey is to be performed,
the Ministry of Education should be contacted to obtain a
listing of all schools with children of
the appropriate age for the survey.
Because the age range for the survey is 6-12 years, the grades in which
these children are likely
to be enrolled
should be determined.
Ideally, the Ministry of Education will have such a listing.
If one nationwide survey is performed, a listing
of schools for the entire
naiton is needed. If
subnational estimates are required, then a listing of the schools for
each subnational area is needed. If
enrolment information for
each school is available, the
PPS method should be used for
selection. If enrolment
information is not available, then systematic
sampling can be performed.
Selecting schools
When
performing school-based surveys in a geographical area, the first questions are:
* Is there a list of all schools in the
geographic area with the appropriate age range?
* If there is a list of schools, is the number of
pupils in each school known?
In
most areas, a list of schools and their respective enrolments is
available. Ensure that
there are the
same number of grades/levels in
the schools. If a list
of schools and enrolments is
available, the selection of schools
should be performede using
the PPS method
described for selection communities. If there is a list of schools but the
enrolments are not known,
schools can be
selected using systematic selection.
Using
sytematic selection, rather than PPS, complicates analysis somewhat. However, if enrolment information cannot
be obtained easily there
may be no alternative. If there
is an extremely large number
of schools in an area, or if a
listing of all schools
does not exist,
another method can be
used. This alternative method is
described later in these guidelines.
Method 1 - schoold when their enrolments are known
In this situation the PPS method for
selection communities, as described earlier
in this Chapter, should
be used. First, generate a list
of schools similar to that shown in
Table 11. Second,
determine the cumulative enrolment.
Finally, select schools using
the same PPS method as described for
selecting communities (see Table 10).
Table
11: Selection of schools using the PPS method
|
School |
Enrolment |
Cumulative enrolment |
|
Utural |
600 |
600 |
|
Mina |
700 |
1,300 |
|
Bolama |
350 |
1,650 |
Method 2 - a list of schools is available, but enrolments are not
known
When a list
of schools is available but the enrolment
of each school is not known, the systematic selection
method should be employed as follows.
* Obtain a list of the schools and number them
from 1 to N (the total number of
schools).
* Determine the number of schools to sample
(n), usually thrity.
* Calculate
the "sampling interval" (k) by N/n (always
round down to the nearest whole integer).
* Usine a random number table, select a number
between 1 and k. Whichever
number is randomly selected, refer to the school list and include that school in the survey.
* Select every kth school after the first
selected school.
Example
of systematic selection of schools
For illustrative
purposes, Table 12 lists fifty
schools. The following method
would be used to select eight schools:
Step
one: There are fifty schools,
therefore N = 50.
Step two:
The number of
schools to sample
is eight; therefore n = 8.
Step
three: The sampling interval is 50 / 8
= 6.25; round down to the nearest
whole integer, which is 6; therefore,
k = 6.
Step
four: Using a random number table,
select a number from 1 to (and
including) 6. In this example, suppose the number selected had been 3.
Accordingly, the first
school to be selected
would be the third school on the
list, which in this example is Bolama.
Step five:
Select every sixth school
thereafter; in this example, the selected schools would be
the 3rd, 9th, 15th, 21st, 27th, 33rd,
39th, and 45th schools on the list.
In
some circumstances, this method might result in the selection of
more than the number needed. In
the above example,
for instance, had the random number chosen in Step four been 1 or 2, then
nine schools would have been
selected rather than eight.
This is because the value for K was rounded down from 6.25 to 6.
In this situation, to remove one school so that
only eight are selected,
again go to the random number table to pick a number.
The school that corresponds to that random number is removed from the
survey.
To
analyse properly the data collected using sytematic sampling, additional information
needed would include
the number of eligible pupils in each school. Note that
usually thrity clusters are selected;
the eight indicatedin Table 12 have been
selected in this example for illustrative purposes only.
Table 12:
Selection of schools
using
the systematic selection method
|
School |
Selected |
School |
Selected |
|
1 Utural |
|
26 Ban Vinai |
|
|
2 Mina |
|
27 Puratna |
Y |
|
3 Bolama |
Y |
28 Kegalni |
|
|
4 Taluma |
|
29 Hamali-Ura |
|
|
5 War-Yali |
|
30 Kameni |
|
|
6 Galey |
|
31 Kiroya |
|
|
7 Tarum |
|
32 Yanwela |
|
|
8 Hamtato |
|
33 Bagvi |
Y |
|
9 Nayjaff |
Y |
34 Atota |
|
|
10 Nuviya |
|
35 Kogouva |
|
|
11 Cattical |
|
36 Ahekpa |
|
|
12 Paralai |
|
37 Yondot |
|
|
13 Egala-Kuru |
|
38 Nozop |
|
|
14 Uwarnapol |
|
39 Mapazko |
Y |
|
15 Hilandia |
Y |
40 Lotohah |
|
|
16 Assosa |
|
41 Voattigan |
|
|
17 Dimma |
|
42 Plitok |
|
|
18 Aisha |
|
43 Dopoltan |
|
|
19 Nam Yao |
|
44 Cococopa |
|
|
20 Mai Jarim |
|
45 Famegzi |
Y |
|
21 Pua |
Y |
46 Jigpelay |
|
|
22 Gambela |
|
47 Mewoah |
|
|
23 Fugnido |
|
48 Odigla |
|
|
24 Degeh Bur |
|
49 Sanbati |
|
|
25 Mezan |
|
50 Andidwa |
|
These results indicate that there is no iodine deficiency,
and that salt iodization is therfore having the
required impact. There
is no evidence of significant overiodization. No
changes are needed on the basis
of these results, but further follow-up
is always essential.
Thyroid size by ultrasonography
Ultrasonography is a safe, non-invasive specialized
technique, which provides a more
precise measurement of thyroid
volume compared with palpation.
This becomes especially significant
when the
prevaelence of visible
goitres is small,
and in monitoring iodine control programmes where thyroid volumes
are expected to decrease over
time. In the future, ultrasonography is poised
to become widely used to assess IDD.
The technical aspectsw of thyroid
ultrasonography are reported in Annex 2.
Feasibility
Portable (weight 12-15 kg) ultrasound equipment with
a 7.5
MHz transducer currently costs
about US $15,000.
A source of electricity is needed,
and the operator needs to be specially trained in the technique.
Interpretation
Results of
ultrasonography from a study
population should be compared with normative data. No universal normative values for thyroid
volume measured by ultrasonography in schoolchildren of iodine sufficient populations are
presently available.
Data from
many countries have emphasized
the importance of establishing normative values for the
popultions being examined. Normative
values for thyroid volume
in iodine replete
schoolchildren
aged 6-15 years should be presented as a
function of age, sex, and body
surface area (BSA) in order to take into account the differences in body
development among children of the same
age in different countries. This
approach was considered potentially useful
in countries with high prevalence
of child growth retardation due to malnutrition with
both stunting (low height-for-age) and underweight (low
weight-for-age).
An
advantage of the thyroid volume-for-BSA is that the age of the child is not
required, which in some populations is
not known with certanity.
A limitation of the thyroid volume-for
BSA is
that it
requires the collection of weights
and heights: in severely malnourished populations of
schoolchildren, 10% or more may have a
BSA below the lowest BSA cut-off of 0.8.
Blood constituents
Two blood
constituents, TSH (thyroid stimulating
hormone or thyrotropin) and
thyroglobulin (Tg) can serve
as surveillance indicators. In a population survey, blood spots on
filter paper or serum samples can be
used to measure TSH and/or Tg.
Determining serum
concentrations of the
thyroid hormones, thyroxine (T4)
and truuiditgtribube (T3),
is usually not recommended for monitoring iodine
nutrition, because these tests are more
cumbersome, more expensive,
and less sensitive indicators.
In iodine
deficiency, the serum T4 is typically lower and
the serum T3 higher than in normal populations. However, the overlap is large enough to make these tests not
practical for ordinary
epidemiological
purposes.
Thyroid stimulating hormone (TSH)
Biological features
The pituitary secretes TSH in response to
circulating levels of T4. The serum TSH rises when serum T4
concentrations are low, and
falls when they
are high. Iodine
deficiency lowers circulating T4
and raises the serum TSH, so that iodine-deficient populaions generally have
higher serum TSH concentratios than do
iodine-sufficient groups.
However, the
different is not great and much
overlap occurs between individual
TSH values. Therefore,
the blood TSH concentration in
school-age children and
adults is not a
practical marker for iodine deficiency,
and its routine use in school-based surveys is not recommended.
In contrast, TSH is neonates is a valuable
indicator for iodine deficiney. The
neonates thyroid has
a low iodine
content compared to that of the adult, and hence iodine turnover is much higher.
The high turnover, which is
exaggerated in iodine deficiency, requires increased stimulation by TSH. Hence,
TSH levels are increased in iodine deficient populations
for the first few
weeks of life: this phenomenon
is called transient hyperthyrotopinemia (32).
The
prevalence of neonates with elevated TSH levels is therefore a
valuable indicator of the severity of iodine deficiency in a
given population. It
has the additional
advantage of highlighting the fact
that iodine deficiency directly affects the developing brain.
In iodine sufficient populations, about 1 in
4000 neonates has congenital hypothyroidism, usually
because of thyroid dysplasia. Prompt
correction with thyroid hormone
is essential to
avoid permanent mental retardation.
Thyroid
hormone affects proper development of the central nervous system, particularly
its myelination, a process that is
very active in the
perinatal period. To
detect congenital
hypothyroidism and initiate
rapid treatment, most
developed countries conduct universal screening of neonates with blood
spot TSH taken on filter papers, or
occasionally with blood spot T4 followed by TSH.
While screening in developed countries is directed
at detecting neonates with TSH elevations which are 20 mlU/ld whole
blood or higher, the
availability of TSH assays
sensitive to 5
mlU/l
permits
detection of mild elevations above normal.
This permits detection of
transient hyperthyrotopinemia. To
be broadly applicable in a
population, the screening must be universal,
and not omit children born in remote or impoverished
areas. For countries and
regions that already have a system
of universal neonatal screening
with a sensitive TSH assay in place, the
data can be examined
and transient iodine
deficiency recognized usually
without further surveying.
Feasibility
Serum TSH
is widely used in the field of
thyroidology as a sensitive
marker for both
hypothyroidism and hyperthyroidism. Methods
for determining TSH
concentrations from either dried whole
blood spots on
filter paper or from
serum, as well established and
widely available. Typically,
a few
drops of whole blood
are collected on filter paper from the cord or by
prick of te heel or other site.
It is essential that sterile equipment be
used, either lancets for blood spot collection or needles
and syringes for collecting whole blood
with which the
serum is separated.
Standard procedures for handling
blood products or objects contaminated with blood should be followed. The risk of contracting HIV or
hepatitis infection from dried blood spots is extremely low.
Blood can be taken either from the cord at delivery
or by heel-prick after birth (usually after 72 hours). Some
experimental data suggest normal
values for cord blood are higher than
those for heel prick blood. Blood
spots, once dried, are stable. They can
be stored in plastic bag and transported even through normal postal systems and are usually for up
to six weeks.
It must
be emphasized that the
primary purpose of
screening programmes is to detect congenital hypothyroidism, and its use
as an indicator of iodine nutrition will
be a spin-off. Hence the only additional cost will be for data
analysis.
It
is not recommended that a neonatal screening programme be set up solely to assess community iodine
deficiency. Less expensive means for obtaining this
inforamtion exist.
TSH screening
is inappropriate for developing
countries where health budgets
are low. In such
countries, mortality among children under five is high due to nutritional
deficiencies and infectious diseases,
and screening programmes for
congenital hypothyroidism are not cost effective.
Performance
A
variety of kits for measuring TSH are available commercially in developed
countries. Most have been carefully
standardized, and perform adequately.
Assays that utilize monoclonal
antibodies,
which
can detect TSH as low as 5 mlU/l in whole blood spots, are more useful for recognizing iodine
deficiency.
Interpretation
Permanent sporadic
congenital hypothyroidism, with
extremely elevated neonatal TSH,
occurs in approximately 1 of 4000 births
in iodine-sufficient countries. Other than infrequent cases of goitrogen
exposure, iodine deficiency is
the only
significant factor to increase this incidence.
The increase in the number of neonates with
moderately elevated TSH concentrations
(above 5 mlU/l whole blood) is proportional to the degree of iodine deficiency. It may be higher than 40% in
severe endemic areas.
Interpretation is complicated when antiseptics
containing beta-iodine, such
as povidone iodine (Betadine
TM), are
used for cleaning the
perineum prior to delivery or even
the umbilical area of
the baby. Beta-iodine increases TSH
levels in the neonate in both cord blood and heel-prick
specimens.
Thyroglobulin (Tg)
Thyroglobulin is
the most abundant protein of
the thyroid, providing the matrix for thyroid hormone synthesis.
Normally, small amounts are secreted or leak from the thyroid
into the circulation. When the thyroid is hyperplastic or injured,
much larger amounts are released.
The thyroid
hyperplasia of iodine
deficiency is regularly associated with increased serum Tg levels. In this setting, it refelects
iodine nutrition over a period
of months or
years. This contrasts to urinary iodine
concentration, which assesses more immediate iodine intake.
Several
studies have shown a good correlation with other markers of
iodine deficiency, particularly
goitre. The laboratory technique is similar to that for TSH and othe
immunoassays. It has been
successfully applied to blood
spots (33), but
this particular application has not yet been developed commercially or
studied further.
Survey methods
Overview
This
book has so far dealt wth what should be measured, i.e. the indicators of process and impact. This chapter describes how to apply these indicators in the field.
First,
it covers methodologies for monitoring the process of salt iodization alone.
Then it deals with monitoring and
evaluating the impact of salt iodization on target communities. In
practice, during surveys at schools or in households, both salt and
the impact indicators can be assessed at the same time (iodine status
assessment).
Attention to survey methods is important. It will help both to ensure
that subjects who are surveyed or
samples which are collected
are representative of the study population, and
that the surveys are carried out as efficiently as possible.
Salt monitoring
An
IDD control programme based on salt iodization clearly cannot succeed unless all salt for human
consumption is being adequately iodized.
Therefore the most important thing to monitor is the
salt itself, and the most important place to monitor it is at the site of
production.
Monitoring iodine content at site of production
Salt monitoring at the site of production is
the responsibility of the
salt producer. It should be done by titration, or
with rapid test kits provided they are backed up by
titration. A modified Lot
Quality Assurance Sampling
(LQAS) scheme is recommended for implementation by
producers (34).
Government
food inspectors or health inspectors should carry out periodic visits to salt production
facilities to check on the in-house
quality control mechanisms.
They should also collect
samples for titration.
Monitoring iodine content at port of entry
Large producers should certify that the salt which
they produce is iodized within a
specified range. Such producers should seek certification by
the International Organization for Standardization (International Standard ISO 9000 series) as an
added guarantee that their salt is satisfactory iodized.
At
the actual point of entry, customs officers can
realistically be expected to check documentation on large consignments
of salt, and visibly inspect
all imports to check that
the salt is suitably packed and labelled. Each consignments should be tested with a
rapid test kit, accepting that this is not any kind
of representative sampling. Suspect salt should be held
at the border. However, it should be noted that salt may be
imported for industrial purposes and is then not covered by
iodization regulations.
Establishing titration laboratories at points of entry
for salt would appear
to be an attractive option, but
is difficult to implement
in practice. Unloading bags from
a lorry or railway wagon to check a consignment
thoroughly is difficult, and only a few
easily accessible bags can be tested.
Staff would have to be specially recruited and laboratories established
at considerable expense.
Monitoring salt at the point of final packing
In
countries where salt is repacked into small packets (500 g, 1 or 2
kg), samples from each consignment should be collected for titration to ensure that the salt is
adequately iodized.
Monitoring salt at the wholesale and retail levels
In many
countries, health inspectors
carry out regular inspections of wholesale and ratail
premises to ensure sompliance with food regulations. During these inspections, samples of food
may be
collected for laboratory
testing. Such inspections therefore provide an ideal opportunity for checking the
salt on sale, testing it with rapid test
kits, and collecting samples for titration.
All salt samples should be carefully labelled before dispatch to the laboratory.
The results
of formal salt monitoring
should be provided
to producers. Where a
specific producer consistently fails
to comply, appropriate legal steps should be taken.
Monitoring salt at the community level
Salt monitoring,
using test kits, should be
conducted at the community
level to ensure that only iodized salt is on sale
to the public. This should be carried out by
environmental health workers, village or community health workers, or others.
Salt samples should be
collected at the
household level during periodic surveys to evaluate coverage
(described below).
Iodine status assessment
Iondine status assessment requires conducting a cross-sectional survey of
a representative sample
of the entire
target population. The recommended
survey method is
multistage "proportionate to
population size" (PPS) cluster sampling
(35). This method has been in use
for many years for the evaluation of
immunization (EPI) coverage, and can be applied to many
other health indicators. The target population for the survey should be
either school-aged children or
women of
childbearing age. Surveys should be either school-based or household-based.
These notes
are intended as a general guide to
the principles underlying the conduct of such surveys. Surveys are
expensive, and the issues of sample size and selection of sites
must be given very careful
consideration. It is recommended
that expert epidemiological help be sought at any early stage in the design of an IDD prevalence survey.
The principal requirement for applying the PPS
method is that a listing (sampling frame) is available of all the
sampling units and their respective
populations. For IDD surveys, the sampling unit
should be either communities or
schools. In the
latter case, a list of the
enrolments (total number of pupils) of
each school is required.
This sampling scheme ensures that larger
communities or schools are more likely
to be selected than smaller ones.
Each selected sampling unit is one cluster. In a defined
geographical area, thirty clusters should be studied altogether to
ensure a
valid prevalence estimate; examining
fewer clusters can
lead to estimates that
differ substantially from
the true prevalence (36).
If a
complete listing of school enrolments
is not available, schools should
be selected on the basis
of simple random sampling. The final result is then adjusted by
weighing the results obtained
usig the number of students actually enrolled in the schools selected for the
survey.
Within each cluster, a specified number of
school-aged children or adult women are
selected for study. Each selected
provides a urine specimen and a sample
of salt from their home.
While the number of samples of each that should be
collected is somewhat flexible,
thirty samples of both urine
and salt per cluster
are generally recommended. Selection of
at least 30
samples allows
for inference at the cluster
levelm i.e. it permits
looking at differences among clusters and
giving an indication of localities where iodine deficiency may
still be a problem.
School-based or household-based surveys?
The school-based PPS cluster sampling method
is recommended as the
most efficient and practical approach for
performing an iodine status or an
IDD prevalence survey. However,
school-based PPS cluster surveys
may not be appropriate
under all circumstances, as
shown in Table 8.
Stratification
One 30-cluster
survey is not sufficient
for all countires, practically those with large
populations or those that are spread over
a wide area.
For example, consider a country
that is divided into
two ecological zones - lowlands
and mountains - where
IDD was previously
only a recognized problem
in the mountains.
In this case, the ecological zones should be
treated separately and surveys carried
out in each. Frequently a country is
divided into subnational units, such as regions or provinces, and each of these
may form the basis for a survey.
Table 8: Circumstances when school-based PPS cluster
surveys may not be appropriate
|
Reason |
Effect |
Recommended action |
|
Low school enrolment or attendance (below 50% of target population). |
Schoolchildren may come from better off families and are then unrepresentative of the general population |
Either: Compare goitre prevalence in children attending school with that in those who are not at significant different proceed with school surveys. Or: Survey adult women or school-aged children in households. |
|
School feeding schemes (particularly if specific micronutrient supplements are included). |
Iodine status of schoolchildren is better than that of the community as a whole. |
Survey adult
women (les than 30
years old) in households. |
|
Low enrolment or girls in schools toward (more than 25% below that of boys). |
Survey is biased towards boys, while girls as future mothers are the most important target group. |
Survey adult women (less than 30 years old) in households, or school-aged children in house- holds and schools. |
|
Another micronutrient deficiency survey at household level is planned. |
Resources are unnecessarily wasted on two separate surveys. |
Combine the two surveys (see 5.4). |
Survey methodology
Thirty clusters are selected from the overall
sampling frame by systematic sampling.
The total population or
total enrolment divided by 30
determines the sampling interval (K).
The starting point of the survey is chosen by selectig a random number
between 1 and K.
In a
school survey, the thirty children selected
for urine collection should be
chosen by systematic random sampling.
Only children between the ages of
6 and 12 years inclusive should be
selected.
Salt samples should also be collected for tiration
at the
same time as the IDD prevalence survey is performed. If
possible, advance notice should be given so that the same children
selected for urine collection
may bring salt to school on
the day in question.
If advance notice is not given, ten
salt samples should be collected
in households in the nearest village to
theschool.
In
a household survey, the team should identify the centre of the chosen community
and there spin a bottle to choose
in which direction households should be selected. Each house
should be
visited acording
to the direction the bottle is
pointing, and either a women or a
child selected in each household (maximum one per household) until the target
number is reached.
Combined micronutrient deficiency surveys
IDD prevalence
surveys may be efficiently combined
with those aimed at assessing the
prevalence of other
micronutrient deficiencies,
such as vitamin A
and iron, or
indeed other
cluster
surveys. The recommended methodology for
such surveys is also PPS cluster sampling.
The sampling
units should be communities,
not schools, since these surveys take place in households
in order to identify women and young children - the most vulnerable groups.
The simplest
way of including an IDD component
is to collect urine
for iodine estimation from the same women or children who are selected for collection of blood for
assessment of vitamin A and/or iron
status, and to ask for a household salt sample at the same time.
Alternatively, the nearest school to
the selected community should
be visited and urine
samples collected as outlined above.
IDD surveys in areas with no prevalence data
When attempting
to answer the question
"does IDD occur?", the selection of
schools or communities for surveying
should be purposive, i.e. on the
basis of IDD being suspected or
predicted in that particular
location. Factors which may lead
to the suspicion that IDD occurs in a particular area are outlined
in Chapter 1. The most useful type of survey for this purpose
is
usually
primary-school based.
Goitre palpation
of each subject takes very little
time. The examination of a
statistically representative number of
children will provide a good picture of the overall IDD status
in the area, and will allow a
reliable assessmet of goitre rates.
This is particularly important
if no estimate of
overall goitre prevalence is
available.
It is
recommended that at least 200 children be examined in a
given school, of the entire enrolment if this is a lower number.
For instance, assessment of total
goitre prevalence in a school of 600
pupils - with 95% confidence and
the same relative precision -
requires examining 83
children if the
estimated overall prevalence is
50%, but 234 children
if the latter percentage is 20%.
In addition, at least 30 childen
should be selected for urine
collection in any school.
Sentinel surveillance
Large-scale,
representative cross-sectional surveys are generally toos costly
to be used
as an instrument
for the regular monitoring of IDD control. To assess
the change in iodine status of a defined
population over time, the method of monitoring which has proved most practical is that done through
the selection of sentinel districts.
Such districts are chosen on the
basis of their being remote and being affected by moderate
or severe IDD prior to the
implementation of the IDD control programmes.
In each selection district, at least three rural
schools should be chosen at random for
surveying. An urban area should
also be included to act as a control, and again at least three
schools should be selected.
In each
selection school in the sentinel districts, urine and
household salt samples should be collected as outlined above. If resources are limited, the number of urine
samples collected per school may be reduced to a total of at least 60
samples in the district as a whole, with 20 from each
school.
Requirement
involve deriving a measure of central tendency and a measure
of variability or
spread of the
distribution. Unfortunately, many
IDD parameters are not normally
distributed.
Rather,
the results may be highly skewed in one direction.
For example, the distribution of both urinary
iodine and tyroid size values
are typically skewed
to the right
(positively skewed). The upper
tail of the distribution is longer than
the lower tail. In
such cases, the use of
means and standard deviations to
summarize the data is inappropriate, and
non-parametric methods should
be used to summarize
and compare distributions.
The
median (which is simplythe middle value of the
distribution) is usead as the
measure of central tendency. The median
is the same thing
as the 50th percentile. Half the
results in the distribution are
above the median and half are
below. It is equidistant from either extreme.
A useful
way of describing a spread
of values which
is not normally distributed involves the use of selected
percentiles. The value
of the 20th and 80th percentiles
(first and fourth quintiles) would be suitable, and
would give a sense of shape to the distribution
of values. However, it
has been customary practice in
giving the results of IDD surveys
to use cut-off points to delineate the lower trail
of the distribution.
For
example, in a frequency distributio of urinary iodine values, it is
helpful to indicate the number and proportion below
set values (typically 100, 50 and 20 ug/l). After iodine prophylaxis
has been
introduced, it may also be
helpful to indicate
the proportion of values above a
particularly high level (e.g. 500 ug/l).
It is important not to overinterpret the results
obtained. For example, it is a common fallacy to say that all
children with a spot urine iodine value
below 100 ug/l are iodine deficient. If
the edian is 100 ug/l, then by definition half of the values will be below
this level. Individual spot urine
iodine values are likely to be highly variable over time.
It should be noted that in carrying out a
survey, only a sample of individuals is examined - not the entire population.
There will therefore inevitably
be a degree of sampling error in the results
obtained. This is decreased - but
not eliminated - by increasing the sample size, but this also
increases cost.
The use of
confidence intervals gives an idea of the
range in which the true population value is likely to lie.
Ninety-five percent confidence intervals can be calculated for a
median, and should be quoted alongside
the value itself.
In
compiling overall results of IDD surveys, e.g. at the national level, it is
important not to simply take averages of subnational data. By so doing, the overall result obtained
may be
biased. Rather, the following guidelines
are useful:
* Results from prevalence surveys in different
regions should be weighted
according to population size,
before combining them.
For example, goitre prevalence
data should be adjusted by the
size of the total study population. The total enrolment of all schools
in the region, or the total
population of the region, should be used to make this
adjustment.
* Urinary
iodine values and thyroid
volumes from ultrasound should be treated in a similar way. (These are
both numerical variables, as
compared to presence or absence of goitre, which is
a
categorical variable).
* Results from sentinel data are not nationally representative data, and therefore should not be presented as such.
Instead, the median of
medians from each sentinel
district should be presented
as the "overall median urinary iodine from x sentinel districts".
Indicators of the sustainable elimination of IDD
In considering
whether the sustainable
elimination of iodine deficiency as a
public health problem has been achieved,
the following criteria should be met (see also Table 9).
With regard to salt iodization, availability and
consumption of adequately iodized
salt (>15 ppm iodine) must be
guaranteed. This is demonstrated by its use by more than 90%
of households.
Preconditions for
the use of this vehicle for
eliminating IDD are:
* Local
production and/or importation of
iodized salt in a
quantity that is sufficient to satisfy the potential human demand (about 4-5
kg/person/year);
* 95% of salt for human consumption must be
iodized according to government
standards for iodine content, at the production
or important levels;
* The percentage of food-grade salt with
iodine content of at least
15 ppm, in a representative sample of households, must be equal to or greater than 90%; and
* Iodine estiamtion at the point of production
or importation, and at the
wholesale and retail levels, must be
determined by titration; at the
household level, it may be determined by either titration or certified kits.
With regard to the population's iodine status:
* The median urinary concentration should be at
least 100 ug/l, with less than 20% of
values below 50 ug/l; and
* The most recent monitoring data (national or
regional) should have been collected in
the last two years.
At least eight out of the following ten
programmatic indicators should occur:
* An effective, functional national body
(council or committee) resposible to the government for the national programme
for the elimination of IDD
(this council should be multidisciplinary, involving the relevant field of nutrition,
medicine, education, the salt
industry, the media, and consumers,
with a chairman appoited by the Ministry of Health);
* Evidence of political commitment to universal
salt iodization and the elimination of
IDD.
* Appointment
of a responsible executive officer for
the IDD eliminatio programme.
* Legislation or regulations on universal salt
iodization (while ideally
regulations should cover
both human and
agricultural salt, if the latter is not covered this does
not necessarily preclude a
country from being certified as IDD-free);
* Commitment to assessment and reassessment of
progress in the elimination of IDD, with access to laboratories able to provide accurate data on salt and urinary
iodine;
* A programme of public education and social
mobilization on the importance of IDD nd the consumption of iodized salt;
* Regular
data on salt iodine at the
factory, retail and household levels;
* Regular laboratory data on urinary data
iodine in school-aged children, with
appropriate sampling for higher risk areas;
* Cooperation from the salt industry in
maintenance of quality control; and
* A
database for recording of results
or regular monitoring procedures, particularly for salt
iodine, urinary iodine and, if
available, neonatal TSH, with mandatory public reporting.
Table 9:
Summary of criteria for monitoring
progress
towards sustainable
elimination of IDD
as a public health problem
|
Indicators |
Goals |
|
Salt iodization Proportion of households using adequately iodized salt |
>90%* |
|
Urinary iodine Proportion below 100 ug/l Proportion below 50 ug/l |
<50%* <20%* |
|
Programmatic indicators Attainment of the indicators specified on the opposite page |
At least 8 of the 10 |
*These goals are expressed
as percentage of population.
Acceptable
iodine status
In
addition to eliminating IDD, acceptable iodine nutrition will be achieved if median urinary iodine is
not greater than 300 ug/l (see also Chapter 2.1).
Partnership evaluatiion
There
is a need for periodic review of the entire programme, with the help
of WHO, UNICEF,
ICCIDD, and other
appropriate organizations. Such external evaluation provides
independent assessment, which is
extremely helpful to a country programme. Partnership evaluation
can also provide
programmes with reassurance of
their performance and effectiveness.
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