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Finding the right MUAC cut-off to improve screening efficiency

Author Koert Ritmeijer, MSF Holland

In Hlaing Thayer township, Yangon, Burma, ORWs were spending a considerable amount of time doing weight and height measurements on all children visited, in order to refer less than 1% to the therapeutic feeding programme. MSF undertook a sensitivity analysis to determine the most appropriate MUAC indicator to use in a first stage screening. This was done to reduce the time spent on the screening excercise without reducing sensitivity.

Introduction

In 1988, the government of Myanmar (Burma) started a resettlement scheme for urban slum populations, from the capital Yangon city to new areas around the city. Hlaingthayar is one of these newly formed satellite townships with a population of 157,000 inhabitants. A majority of those living in Hliangthayar are very poor and rely on daily wages as day labourers, factory workers, food vendors and other informal sector jobs in Yangon. Resettlement to the township has increased their vulnerability. Essential infrastructure in these townships like water supplies and sanitation works are poorly developed, and during the monsoon rains large parts of the township are flooded. Basic health care in Hlaingthayar is poorly developed and is characterised by chronic shortages of staff, drugs and medical materials. The main health problems include high maternal mortality, high childhood morbidity and mortality and high incidence of STD/HIV and epidemic diseases (cholera and Dengue). Nutrition surveys carried out by MSF in 1992, 1993 and 1996 in the township revealed consistently high levels of both acute and chronic malnutrition amongst children under five years of age.

MSF began implementing an urban health care programme in Hliangthayar in 1993. The programme focuses on women and young children and includes the running of independent MCH clinics, feeding centres, community outreach programmes, AIDS/HIV awareness programmes, water and sanitation initiatives, and supplying essential drugs to public (government) health facilities.

Problem

At the start of MSF's programme, Outreach workers (ORWs) conducted screening during their home visits, referring children with a Mid Upper Arm Circumference (MUAC) less than 135mm to the feeding centre for a weight for height assessment. At the feeding centre children with a 'weight for height' < 75% of the median in the reference population1 were admitted. The admission criterium of 75% W/H was chosen for pragmatic programme capacity as well as 'economy of scale' reasons. The feeding programme did not have the capacity to admit both severely (<70% W/H) as well as moderately malnourished children (70-79% W/H). However, on
the other hand, only admitting severely malnourished would lead to feeding centres with only very limited case loads (20-30 children per feeding centre). For efficiency reasons children between 70 and 74% W/H were also admitted as a risk group "to fill up" the capacity of the feeding centres.

Although this MUAC screening had a sensitivity2 of some 85% the positive predictive value3 was very poor. Only 5% of children with a MUAC <135 had a weight-for-height <75% of the median. Therefore most mothers and children referred to the feeding centre for weight for height check ups were sent home again. This wasted a lot of time for mothers, undermined credibility of the programme and led to loss of motivation amongst the ORWs. At the time it was also felt that while using a stricter MUAC screening cut-off would improve the positive predictive value, it would also significantly reduce sensitivity, leading to a situation where a high proportion of truly malnourished children would be missed. As a result, MSF decided to let ORWs conduct weight-for-height measurements of all children under five during their home visits in Hlaingthayar so that they could refer malnourished children to therapeutic feeding programmes.

Currently all children under 75% weight for height are admitted to the feeding centres. Roughly 3,900 children are screened every month, of whom 20-30 are referred to the feeding centre (0.5-0.8%). As it takes on average 7.5 minutes per child to do a weight for height measurement, the total time spent by ORWs each month on nutritional screening to detect 20-30 cases amounts to almost 500 hours. In order to increase the efficiency of the screening, MSF has recently been considering reverting to MUAC measurements as a first screening tool, so that more time could be made available for the ORWs to undertake other priority tasks, e.g. health education.

Objective and Method

The task faced by MSF was to select the most appropriate MUAC cut-off point for initial (first stage) screening of children under five, so as to reduce the number of unnecessary weight and height measurements. This required analysis and calculation of the sensitivity, specificity and positive predictive value of different MUAC cut-off points, in identifying children less than 75% weight for height for admission to therapeutic feeding. However, as comprehensive MUAC data for the Hlang Thayar population were not available, nutrition survey databases from other populations with a similar global prevalence of malnutrition were used. These data were from Rohinga refugees in Bangladesh between 1992-3 and the urban population n Malange, Angola in 1994.

Table 1.Comparison of prevalence using weight for height cut-offs from nutrition survey databases. Survey Database
Survey Database Hlaing
(1996)
Bamgladesh
(1992/93)
Angola
(1994)
sample size 938 1715 1147
(6-59 months)
global malnutrition rate
(W/H <-2 Zsc)
12.8%
12.2%
11.9%
severe malnutrition rate
(W/H <-3 Zsc)
1.5%
0.4%
1.5%
'admission Criteria'
(W/H <75% Med.)
3.4%
1%
2.2%
MUAC < 135 mm
28.7%
41.9%
MUAC < 130 mm
15.9%
27.6%

 

Table 2. Sensitivity specificity and positive predictive value analysis
Bangladesh
(Rohingas)
Angola
Admission Criteria
(W/H <75% Med.)
MUAC<135mm MUAC<130mm MUAC<135mm MUAC<130mm
Sensitivity
88.2%
82.4%
92.0%
84.0%
Specificity
71.8%
84.7%
59.2%
73.7%
Positive predictive value
3.0%
5.1%
4.8%
6.6%

Analysis of the data

In multiple database analyses there was a limited correlation between MUAC and weight-for-height (r=0.6) and a generally poor positive predictive value of MUAC cut-off points of 135 and 130 mm in identifying children < 75% weight for height (see table2). This confirmed that MUAC on its own was an inappropriate screening tool and would need to be used in combination with weight for height measurements. However, the data also showed that MUAC cut-offs of 130 and 135 mm may be considered sufficiently sensitive to identify children under 75% weight for height, but that the specificity of a MUAC cut-off of <130 mm is significantly higher than a MUAC of <135 mm. Although the positive predictive value of a MUAC cut-off of 130 and 135 mm for a weight for height of <75% remains poor, considerable efficiency gains can be made by using MUAC as a first screening stage. The total time saved on the whole screening procedure can amount to 65% using a 130 mm cut off and 52% using a 135 mm cut off. The number of false negatives by MUAC screening (children with a weight for height <75% but not selected by MUAC screening) remains acceptable. Of the 20-30 children currently referred each month using weight for height screening, an estimated 3-5 children (17%) would be missed each month when using a MUAC of < 130 mm. Further analysis shows that these false negatives do not include severely malnourished children (weight for height < 70%) but rather "borderline" cases with a weight for height between 72-75%.

Table 3 Cost-efficiency comparison of different nutritional screening strategies (estimations, based on extrapolation of average approximations
Screening strategy
(3900 children per month)
Total monthly time sent on screening
(hrs.)
Time saved Children missed (false negatives by MUAC)
MUAC
(1min/chid)
MUAC
(7.5min/child)
Total
(MUAC+ W/H)
1. W/H screening of all children (=3900 children) 0hrs 488 hrs 488hrs 0 0
2. MUAC of all children + W/H screening of children of children with MUAC <135mm (3900x0.35=1365 children) 65hrs 171hrs 236hrs 52% 10%
3. MUAC of all children + W/H screening of children with MUAC <130mm (3900x0.22=860 children) 65hrs 107hrs 172hrs 65% 17%

Recommendations

Our final recommendation has been to revert to a two-stage nutritional screening strategy involving;

1. MUAC screening of all children under five.
2. Weight for height screening of all children with a MUAC <130 mm.

Show footnotes

1For explanation of terms see 'Z-scores' article in Field Exchange Issue I

2The term sensitivity means the ability of the indicator (in this case a MUAC of < than 135 mm) to identify correctly children less than 75% weight for height. In other words a sensitivity of 85% means that 85% of children with a weight for height less than 75% would also have a MUAC of less than 135 mm.

3The positive predictive value of the MUAC screening test is the true positives as a percentage of all those tested positive by the screening method., i.e. those with weight-for-height < 75% as a percentage of all children tested with a MUAC < 135mm

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Koert Ritmeijer (1998). Finding the right MUAC cut-off to improve screening efficiency. Field Exchange 4, June 1998. p23. www.ennonline.net/fex/4/finding

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