MUAC Versus Weight-for-Height in Assessing Severe Malnutrition
Summary of published paper1
Current WHO guidelines for the management of severe malnutrition in children recommend calculation of weight for height z score (WHZ) or % of the median (WHZ)2. However, in practice, this can be difficult to implement in resource-poor settings in sick children. A recently published study set out to evaluate MUAC and visible severe wasting3 as predictors of inpatient mortality at a district hospital in sub-Saharan Africa and to compare these with WHZ.
The principle aim of the study was to examine the predictive value for inpatient death of MUAC compared with WHZ among children aged 12 to 59 months. In addition, the study:
- investigated whether there were any differences in the children identified by MUAC versus WHZ
- evaluated the clinical sign of visible severe wasting as a predictor of subsequent inpatient death, and
- evaluated MUAC as an indicator of the presence of a WHZ less than or equal to -3.
The study was conducted at Kilifi District Hospital, located in a rural, malaria-endemic area on the Kenyan coast. Approximately 10% of women attending the hospital antenatal clinic were infected with human immunodeficiency virus (HIV) in 2000. Antiretroviral therapy was not in routine use at the time of the study and data on individual HIV status was not available. Data were prospectively collected from all paediatric admissions as part of an ongoing surveillance study. For this study analysis, data from all children aged 12 to 59 months admitted between April 1, 1999, and July 31, 2002 were included.
From April 1 1999, trained clinical assistants measured MUAC, weight and height/length and from September 1, 1999, data on visible severe wasting4 was also collected. Children with a clinical diagnosis of severe malnutrition were treated according to WHO guidelines and local protocols.
WHZ, weight-for-age z score (WAZ), and height-for-age z score (HAZ), using the NCHS reference standards, were calculated using EpiInfo version 6.045. Only children with complete data for all three indices were included in the main analysis.
The predictive value for inpatient death was determined as the area under the receiver operating characteristic (ROC) curves with 95% confidence intervals (CIs) using the roctab and roccomp (a2 test) commands in STATA version 8.0 (Stata Corp, College Station, Tex).
The sensitivities6 and specificities7 of commonly used cutoff values were investigated, as well as the clinical data for differences between children identified as severely malnourished by the MUAC and WHZ methods. Since the WHO recommends that children be treated for severe malnutrition if they have severe wasting or kwashiorkor, the investigators evaluated the positive and negative likelihood ratios for death for each of MUAC, WHZ, and visible severe wasting combined with (and/or) kwashiorkor.
To determine the independent associations of different nutritional indices with mortality, multivariable logistic regression was used that included age, sex, visible severe wasting, WHZ, and MUAC.
To determine the clinical features that differed between children identified by MUAC and WHZ, backward stepwise multivariable logistic regression were used. All analyses were performed using STATA version 8.0, and P<0.05 was used to determine statistical significance.
A total of 8,500 children aged 12 to 59 months were admitted during the study period (1999- 2002). Anthropometric data were incomplete in 3.6% (n=310) of children, who were mostly admitted in extremis and were excluded from the analysis. Overall, 4.4% (n=359) of children included in the study died while in the hospital. Sixteen percent (1282/8190) of admitted children had severe wasting (WHZ<-3) (n = 756), kwashiorkor (n = 778), or both.
Prediction of inpatient mortality
The case fatality rate among children admitted with WHZ less than or equal to -3 was 19.9% (151/756). The case fatality rate among admitted children with MUAC less than or equal to 11.5 cm was 19.0% (166/873) and did not significantly vary with age.
The areas under the receiver operating characteristic curves for predicting inpatient death did not significantly differ between MUAC (0.75, 95% confidence interval, 0.72-0.78) and WHZ (0.74, 95% confidence interval, 0.71-0.77) (P = 0.39).
Sensitivity and specificity for subsequent inpatient death were similar for WHZ, MUAC and visible severe wasting: 46% and 91%, respectively, for MUAC less than or equal to 11.5 cm, 42% and 92% for WHZ less than or equal to -3, and 47% and 93% for visible severe wasting. However, the 3 indices identified different sets of children and were independently associated with mortality.
Clinical features of malnutrition were significantly more common among children with MUAC less than or equal to 11.5 cm, than among those with WHZ less than or equal to -3.
Visible severe wasting was present in 9.0% (608) of 6727 children assessed. The median age of children with visible severe wasting was 24 months (interquartile range, 18-35 months). Of the 608 children with visible severe wasting, 22.5% (137) died, compared with 2.5% (153/6117) without this sign (sensitivity, 47%; specificity, 93%).
The positive and negative likelihood ratios for death for WHZ less than or equal to -3 and/or kwashiorkor were 4.36 (95% CI, 3.95-4.84) and 0.47 (95% CI, 0.41-0.53), respectively; those for MUAC less than or equal to 11.5 cm and/or kwashiorkor were 5.12 (95% CI, 4.49-5.84) and 0.59 (95% CI, 0.54-0.65); and those for visible severe wasting and/or kwashiorkor were 5.31 (95% CI, 4.71-5.97) and 0.46 (95% CI, 0.40-0.53).
A multivariable logistic regression model adjusted for age and sex showed that MUAC, visible severe wasting, and kwashiorkor were all independently associated with inpatient death.
Predicting the current WHO criterion for severe wasting
For detecting the WHO standard criterion for severe wasting (WHZ -3), the sensitivity and specificity of MUAC less than or equal to 11.5 cm were 65.1% (486/746) and 94.8% (7057/7444), respectively. The sensitivity and specificity of visible severe wasting for WHZ-3 were 52.6% (320/608) and 95.3% (5831/6119) (P<.001), respectively. Of 608 children with WHZ less than or equal to -3, 29.3% (n=178) did not have visible severe wasting or MUAC less than or equal to 11.5 cm.
Differences in children identified by MUAC and WHZ
Comparing children with a MUAC less than or equal 11.5 and those with a WHZ <-3 (univariate analysis), the MUAC group were more likely to be stunted, female, and to have a longer history of illness, cough, diarrhoea, subcostal indrawing, visible severe wasting, kwashiorkor, moderate anaemia, and bacteremia.
Although the median ages appeared similar, the distribution of ages also significantly differed between the 2 groups.
Multivariable analysis showed that skin/hair changes associated with recent kwashiorkor, bipedal oedema associated with current kwashiorkor, stunting, subcostal indrawing, no history of seizures, female sex, and younger age were independent associations of having MUAC less than or equal to 11.5 cm, rather than WHZ less than or equal to -3.
The study found that MUAC performed as well as WHZ in predicting inpatient mortality in this context. Since MUAC is inexpensive, more commonly available, does not require a chart to calculate, and is easier to measure than WHZ, the authors suggest it may be a useful screening tool for such children. However, there were differences in the groups of children identified by these methods, and they independently predicted inpatient death. The study observed statistically significant, independent associations of age and sex and identification by MUAC alone, compared with WHZ alone, when adjusted for the effects of other variables. Since the case fatality rate for MUAC less than or equal to 11.5 cm was consistently high (19.0%) at all ages, the authors suggest that an unadjusted MUAC may be clinically useful in this setting.
The authors suggest that stunting is a risk factor in itself for poorer outcomes, and small physical size may contribute to a low MUAC measurement (but has no influence WHZ). In this study, the prevalence of stunting in children with MUAC less than or equal to 11.5 cm was greater than in those with WHZ less than or equal to -3. They also suggest that the association of a number of other clinical characteristics, e.g. bipedal oedema and skin/hair changes among children with MUAC less than or equal to 11.5 cm, may mean MUAC is a better indicator of severe malnutrition than WHZ in this setting.
Visible severe wasting was as useful as anthropometry in this study in detecting severe acute malnutrition. The authors suggest subjective clinical assessment by trained staff in this setting is appropriate, ideally supported by an objective measure such as MUAC, to allow standardization between centres and classification of the degree of malnutrition.
Since visible severe wasting did not predict WHZ less than or equal to -3 as well as MUAC, MUAC may be better in identifying less severely ill children in need of nutritional rehabilitation.
The main limitations of this study are that it was performed at only one site and that varying malaria transmission and HIV prevalence may influence observations. Also, there was no systematic follow-up of deaths post-discharge.
Given their findings, as well as cost and practicalities, the authors suggest that MUAC may be more appropriate than WHZ for identifying severe malnutrition in children aged between 1 and 5 years who are admitted to an African district hospital. However, an assessment that includes MUAC, WHZ, and visible severe wasting increases the number of at-risk children who are identified on admission and highlights those in overlapping groups who are at the greatest risk of dying. They suggest further studies are needed to evaluate MUAC and visible severe wasting in other operational settings and other situations in which anthropometric assessment is difficult to perform.
1Assessment of Severe Malnutrition Among Hospitalized Children in Rural Kenya. Comparison of Weight for Height and Mid Upper Arm Circumference. Berkley J, Mwangi I, Griffiths K, Ahmed, I;Mithwani S, English M, Newton C, Maitland K. JAMA. 2005;294:591-597.
2The WHO defines severe malnutrition requiring hospital admission as weight-for-height z scores (WHZ) of less than or equal to -3 or as less than or equal to 70% of the reference median using US National Centre for Health Statistics (NCHS)/WHO reference values (severe wasting) or symmetrical oedema involving at least the feet (oedematous malnutrition, kwashiorkor).
3Because of the recognised difficulties of measuring weight for height, the WHO Integrated Management of Childhood Illness programme for primary-level care makes use of the clinical sign of visible severe wasting.
4Muscle loss manifested as a wasting of the gluteal area and as the presence of a bony prominence over the chest wall.
5Centres for Disease Control and Prevention, Atlanta
6Sensitivity was defined as the number of inpatient deaths among children with anthropometric measures equal to or below a cutoff value, divided by the total number of inpatient deaths
7Specificity was defined as the number of children discharged alive with anthropometric measures above a cutoff value, divided by the total number of children discharged alive.
Taken from Field Exchange Issue 26, November 2005