Implications of 65 cm height cut-off as age proxy in Bangladesh

Summary of published research¹

Location: Bangladesh

What we know: Age data for children aged 6 to 59 months is often not available in poor, migrant or conflict affected communities. Height cut-offs are often used as a proxy for age - 65 cm for 6 months of age and 110 cm for 59 months of age. No adjustment is recommended for cut-offs in stunted populations.

What this article adds: In a context of prevalent stunting (urban slum in Bangladesh), 12% of children aged 6 –59 months presenting to primary health care centres were misclassified for age using the height cut-off of 65 cm as age proxy, and thus were excluded from nutritional assessments. The majority (97.5%) of the exclusions occurred among children aged 6–17 months, an over-represented age group in the sample compared to population demographics. There is a need for research on context specific adjustment of height cut offs. There are questions regarding interpretation of GAM prevalence rates by age group in stunted populations and a need to further investigate misclassification risk at 110cms cut off.

Age documentation for children is often not available in poor communities (that do not record birth date/home births are common), among migrant populations and in the context of natural disasters and conflicts. To enable nutritional assessment in the absence of age data, non-governmental organisations (NGOs) such as Médecins Sans Frontières (MSF) use length/height cut-offs as a proxy for age. A standardised height measuring stick was developed to facilitate this with a lower height cut-off of 65 cm (a proxy for 6 months of age) and an upper cut-off of 110 cm (a proxy for 59 months of age)².

In the complex context of Kamrangirchar slum in Bangladesh, age verification of children during household nutritional screening by MSF was a challenge. The majority of the parents were absent or seeking work, and the caregivers were often family members or neighbours who didn’t know the real age of the child. To solve this practical problem, height cut-off thresholds were used as a proxy for age. As growth stunting is prevalent in Bangladesh, MSF hypothesised that a sizable proportion of children aged ?6 months would be unlikely to have attained a height of 65 cm and would thus be excluded from nutritional assessment. This is of particular concern, as these children are more vulnerable to malnutrition and related mortality risks. Using data from primary health care centres, where age and height parameters were well recorded, an MSF team assessed the proportion of children aged between 6 and 59 months who would be excluded from sampling using the height cut-off of 65 cm as a proxy for age ?6 months. The findings are summarised here.

Method

This was a secondary data analysis of routine primary health care (PHC) data, and included all children aged 6–59 months who attended the clinics for the first time between July and September 2011 in the Kamrangirchar slum. Kamrangirchar has a population of 400 000 living in an area of 3.1 km2. The population is mainly formed of migrants, and the majority of the mothers work. There are no governmental health services in the slum and PHC services are outsourced to NGOs. MSF PHC services are offered at two centres and focus on childhood malnutrition. All children presenting to the PHC for both preventive and curative care had their age, sex and anthropometric measurements recorded. Height/length was measured using a wooden stadiometer, with a precision of 0.1 cm. Weight was measured using a hanging scale, with an accuracy of 100g. Mid-upper arm circumference (MUAC) was measured with a tape measure. In each PHC centre there were two anthropometric measurers who had formal training in the measurement techniques. On-the-job supervision and refresher training were conducted at regular intervals to limit errors in measurement. Ages were verified using birth certificates, vaccination cards and local calendars of events.

Data obtained from the PHC register on height, age and sex were entered into EpiData version 3.1 (EpiData Association, Odense, Denmark) between November and December 2011. A sample size was calculated based on the hypothesis that 6% of the children attending the PHC aged 6–59 months were <65 cm in height. To detect the above with a 95% The required sample size was calculated to be 2072 children (95% confidence interval (CI) and 5% error). The study team determined the proportion of children aged 6–59 months who had not attained a height cut-off of 65 cm (95% CI and 5% error). To assess the representativeness of the age distribution of the study sample, data on the age distribution of children included in the MSF annual nutritional survey in Kamrangirchar in 2011 were compared with the study sample. Data entry was validated by comparing randomly selected entered data with the register; the data were then cleaned of errors and aberrant records.

Results

Data on 2,072 children were available in the electronic database. Twelve children were excluded from the analysis: nine were missing records on height and three had aberrant records (two with height and one with age). Of the remaining 2,060 children included in the analysis, 1,042 (51%) were males and the median age was 24 months. The median height was 78 cm, with a standard deviation of 12.1 (range 50–109). On comparing the age distributions of the children included in the study sample with those included in the nutritional survey, which was based on a random cluster method, there was a significant difference between the age groups 6–17 and 42–53 months. Of the total sample, 240 (12%, 95%CI 10–13) children aged between 6 and 29 months had a height <65 cm. The majority of these children were females (59%) and aged between 6 and 17 months.

The study confirms the hypothesis that, in a context of prevalent stunting, 12% of children aged 6 –59 months are misclassified for age using the height cut-off of 65 cm as age proxy, and are thus excluded from nutritional assessments.

An important limitation of the study is that there might be an issue of representativeness of the sample, as it used PHC data; access to the PHC and morbidity might have an influence on the sample. Notably, the majority (97.5%) of the children who did not attain 65 cm in height were those aged 6–17 months. This age group constituted 35% of the study sample in comparison with 20% of the community survey. This might be explained by the fact that younger children tend to be more vulnerable to morbidity episodes, and as such often constitute a higher proportion of children attending PHC centres. On the other hand, repeated morbidity—which is more likely in the sample that presents to PHC centres—could influence retarded growth and thus increase the proportion of children in the age group 6–17 months, as seen in the PHC sample. This notwithstanding, the overall proportion of exclusion using the current height proxy of 65 cm is still too high.

Discussion

The study authors posit a number of key findings. First, about one in ten children who should undergo nutritional assessment in infancy were excluded—the majority (97.5%) of the exclusions occurred among children aged 6–17 months, who are the most vulnerable age group for malnutrition-related morbidity and mortality. This finding also implies that standardised WHO growth chart data, used as the basis for determining proxy heights for age, are inappropriate in such contexts of prevalent stunting. Second, the findings beg the following question: what should be the ideal height proxy for 6 months of age in this setting? This would require a community cluster survey and validity analysis to determine cut-offs with the highest positive predictive value. Due to height stunting, the upper height cut-off of 110 cm as a proxy for 59 months of age might also misclassify children aged >59 months as being below that age, as they might not have attained a height of 110 cm. This will unnecessarily include children who are not in the target group for nutritional assessments. Here too, determining the ideal upper cut-off as an age proxy of 59 months will require further research. Third, and most importantly, the use of height cut-offs as proxy for age is likely to introduce bias in nutritional surveys assessing malnutrition prevalence in the age group 6–59 months. This could have major public health implications, as nutritional interventions are guided by prevalence rates of malnutrition among such age groups. Fourth, the findings also highlight the need for national growth charts in a country such as Bangladesh, which has a high prevalence of stunting.

¹Ali. E et al (2012). Does the 65 cm height cut-off as age proxy exclude children eligible for nutritional assessment in Bangladesh? International Union Against Tuberculosis and Lung Disease Health solutions for the poor VOL 2 NO 4. Published 21 December 2012. http://dx.doi.org/10.5588/pha.12.0037

²These cut-offs were sourced from the 2006 World Health Organization (WHO) growth standards, which were derived from an international sample of healthy breastfed infants and children raised in an environment with minimal constraints to growth. The height cut-off value of 65 cm is equal to the −1 Z-score of a sex-combined height-for-age curve at 6 months of age.