We have shown that among children admitted to a rural Kenyan district hospital; both MUAC and WFLz were altered by rehydration. Among children who were rehydrated by 10% or more of their admission body weight, the mean change in MUAC was 3.6 mm and in WFLz was 1.4 z scores. We also found that amongst all children with signs of dehydration, approximately one in five children who were classified as severely malnourished at admission by either MUAC or WFLz were classified as non-severely malnourished after 48 hours. Amongst those who gained weight during the 48 hours, approximately one in three became non-severe using WFLz compared to one in four using MUAC.
Since MUAC decreased in children who lost weight and increased proportionately with the degree of rehydration among those who gained weight over a short period of time, we believe that these changes were mostly due to changes in hydration.
There is a paucity of published literature describing changes on diagnostic criteria for SAM among acutely ill hospitalised patients and none has previously reported a repeated measures approach among children. In a study of adults with acute gastroenteritis, anthropometric measures at admission and at 4 weeks post discharge indicated a significant change in both weight and MUAC . Similar to our findings, the authors concluded that the anthropometric changes observed were explained by dehydration and not a change in nutritional status.
Our findings have important implications for the identification and management of individual children with severe malnutrition and for estimations of the prevalence of SAM among hospitalized children. They indicate that such studies may overestimate the true prevalence of SAM and may potentially confound associations if dehydration is also associated with the outcome studied. Since many deaths occur early in admission  this is not easily resolved by using later measurements.
A recent study among severely wasted children (6 to 36 months) with cholera in Bangladesh (mean admission WFLz -3.09 and MUAC 11.3 cm)  reported an average weight gain of approximately 11% at 72 hours post admission. Anthropometric changes during this period were not evaluated. However, if our findings are generalisable, then the 11% weight gain observed in that study would equate to an increase of ~1.5z scores in WFLz after hydration. Many of the children may not have fulfilled the WHO criteria for severe acute malnutrition at admission had they not been dehydrated.
Our findings may reflect the hospital setting and may differ in a village or community context where both the severity of illness and access to services vary. It is unknown to what extent children with poor fluid intake and ongoing losses might become more dehydrated during a long walk to access clinic services.
In our study, a quarter of the children lost weight suggesting further water loss over the observation period. This did not alter our models for percentage change in anthropometric indices. However, when these children were excluded, the proportion who changed nutritional status classification was markedly increased. The difference was especially evident for moderate malnutrition assessed by WFLz compared to MUAC. It is recognised that signs of dehydration, including sunken eyes and delayed skin pinch which were the most common signs in our study, may be unreliable indicators in malnourished children . It is therefore possible that dehydration could be over-diagnosed among malnourished children. This study was not designed to discover whether this was due to children not taking the prescribed amount of fluid, or whether strategies for rehydration were effective.
A strength of this study was in assessing the inter-observer reliability of the two observers before they were involved in data collection thus increasing the trustworthiness of the results. It is notable that for inter-observer reliability, the lowest ICC estimate was for the WFLz measurements. One limitation of this study is that we used a 2 mm scale MUAC tape instead of a 1 mm graduated tape, which limited the precision of absolute MUAC changes. The use of percentage weight change as a measure of hydration may also be a limiting factor as food, drinks and passing stool or urine shortly before measurement may also have altered weight. More precise estimation of hydration including isotope dilution or bioelectrical impedance methods could be used in future.