In view of the increased mortality of children concurrently wasted and stunted, this analysis sought to better understand the prevalence and burden of concurrent WaSt in Mozambique, and to test the applicability of considering MUAC in the case-definition of WaSt, since this was never done before in the published literature, and whether WaSt could be diagnosed with a single measurement.
Overall, the original and proposed case-definitions identified a different number of children with WaSt, being the proposed case-definition with a considerably higher number of children with WaSt. This was as expected as published studies assessing the difference between WHZ and MUAC show a non-concordance between the two measurements: in locations studied, the two measures usually identify two sets of cases with some degree of overlap [15, 17, 18, 32]. In Mozambique, WHZ and MUAC rarely agree in wasting diagnostic classification (Cohen’s Kappa = 0.353, ρ < 0.001) with no change by province, although there is a positive correlation between WHZ and MUAC . For this reason, even where prevalence by WHZ is higher than by MUAC, if MUAC is not considered in the WaSt definition, some wasted children will be excluded.
The patterns of concurrent WaSt from this analysis are similar to those reported by others, where younger children are more affected than older children. Khara et al., found that children aged 12–24 months and 6–19 months were more affected by WaSt with a prevalence of 4.2% . Garenne et al., found that proportion of WaSt showed a “fast increase from 6 to 18 months, a peak around 19 months (12%)”  and started to decline after 24 months old and becoming negligible after 48 months. Schoenbuchner et al.,, on the other hand, reported that the number of children with WaSt started to decline from 12 months of age for girls and 16 months of age for boys . In this study, perhaps due to the inclusion of MUAC, concurrent WaSt was high among children aged 6–17 months with 5.0 and 5.4% for boys and girls respectively, compared to the original case-definition, and among children aged 18–29 months (3.5% for boys and 4.1% for girls) (Figs. 1 and 2). Moreover, with the proposed case-definition there was a greater increase in the prevalence among girls compared to boys with the addition of MUAC (Table 2). This is as expected and likely to be explained by the tendency of MUAC to identify more wasting among girls. A recent study in Mozambique found that girls are 1.6 times more likely to be diagnosed with wasting using MUAC than boys . As for the patterns reported in Table 1 and Figs. 1 and 2, it could be linked to two factors, both related to MUAC: (a) the known age-bias in MUAC when there is an unbalanced age distribution (more younger children in the sample than older), however this is not the case for the sample used in this analysis; (b) the increased likelihood of a younger child being diagnosed with wasting by MUAC in Mozambique, which is 5.3 times more likely in children < 24 months compared to 2.3 times more likely when using WHZ for the same age group .
Our analysis suggests that MUAC should be considered and included in the case-definition for concurrent WaSt by using the case-definition proposed in this study. This would ensure that all wasted children, whether identified by WHZ or MUAC, are included in the case-definition. This is important from both a programmatic and an advocacy standpoint, to ensure that no WaSt children are being excluded from prevalence and burden estimates, especially in contexts without supplementary feeding programmes. In light of the recommendation provided by Myatt et al., that says that “(…) therapeutic feeding programs should cover WaSt given the high mortality risk associated with this condition” , it is important to calculate and report prevalence of WaSt alongside other anthropometric parameters traditionally reported in surveys, in order to ensure that these high-risk children are being identified, targeted and referred to nutrition programmes for treatment and/or counselling, and so that more information about the condition can be obtained. Since mortality risk associated with the WaSt proposed case-definition has not been tested, we recommend that the mortality risk of WaSt original and WaSt proposed case-definitions is analyzed with historical cohort data. As known, MUAC < 125 is associated with an increased risk of mortality  and the inclusion of MUAC will identify many children at high risk of near term mortality.
In our analysis we assessed how best to diagnose WaSt. Our findings show that current wasting treatment programmes in Mozambique, called the Programa de Reabilitação Nutricional (Nutrition Rehabilitation Program in English)  already allows for identification of most of the children with WaSt, as both MUAC and WHZ are used as methods for admission and both severe and moderate wasting cases are enrolled. With this, 100% of children with WaSt can be identified at health facilities, provided that conditions (equipment, trained staff) for MUAC and WHZ measurements are available and assuming that staff do, in fact, use both measures for each child. That is, both MUAC and WHZ must always be measured in all children aged between 6 and 59 months (Additional information 4) presenting to the health facility. Mass MUAC screening at community level, which does not include WHZ, would identify approximately 79.4% of children with WaSt in Mozambique. Using a single cut-off, that is increasing the MUAC threshold or using WAZ to identify the remaining 20% of children at community level where WHZ is not possible to measure, is not cost effective. This is because too large number of false positives, who are not WaSt, are identified (Additional file 5 and Additional file 6 for more details). We carried out an inspection (not reported) to assess if the FPV cases were in reality positive using cut-offs identified in this study and, we found that they were indeed all truly negative. Therefore, the question on how best to identify the 20% of WaSt children missed at community level remains. Our analysis suggests that using a combination of WAZ and MUAC in communities would be more cost-effective, however this adds an additional measure and layer of complication at community level. A cost-effective approach to treatment of malnutrition should identify children most at risk of death, which are those with severe wasting and those with WaSt. For this, a combination of WAZ and MUAC might be able to identify most of the severe wasting by MUAC/WHZ and the WaSt cases without adding too many false positive children who are not in need of treatment. More research is needed to investigate this further. Screening criteria should be judged based on the ability to identify wasted and WaSt children, however if WaSt by MUAC has higher mortality, it will be important to include MUAC in the case definition to judge the screening criteria.
Since most children with WaSt are already included in existing treatment programmes in Mozambique, it is necessary to better understand how children with WaSt can best be treated, and how they respond to treatment. These questions have been addressed in a retrospective cohort study by Odei Obeng-Amoako et al., using treatment records of children admitted to nine outpatient therapeutic programs in Karamoja, Uganda . Results showed that children with WaSt can be treated within existing wasting treatment programs as proposed, nevertheless, more research is needed to analyze the patterns that influence optimal treatment outcomes and response among children with WaSt. As observed from that study, children with WaSt had slightly faster weight gain than children who were wasted alone, however the WaSt recovery rate was low, hence the number of children with WaSt discharged (under the normal CMAM criteria) as non-responders was high . According to Odei Obeng-Amoako et al., the outcome seemed to be linked to a high prevalence of infectious disease in Karamoja and/or the fact that there are low muscle mass stores in children with severe wasting as reported by Reeds et al., in the 1978  as well as indirect evidence suggesting that “muscle mass is also reduced in relation to body weight in stunted children” . We believe that all this information points to a need to ascertain further the factors that influence recovery of children with WaSt, and to what extent this can be quantified by the muscle mass composition of children with WaSt and children without WaSt, differences in fat stores, if children were suffering from an infection or not, with further analysis to assess how each factor separately or together reduces the likelihood of recovery. Understanding the influence of these factors could inform programme adjustments to achieve a good recovery and response rate. Considering the existence of programmatic data, both in emergency and non-emergency settings, those could be used to address these questions, including whether cases identified with the proposed WaSt case-definition respond to standard wasting treatment protocols. It would also be important to estimate an incidence correct factor for WaSt to inform caseload calculations for better programme planning.
Our analysis was subjected to the following limitations: (1) the lack of an agreed gold standard case-definition of concurrent WaSt. (2) We do not have evidence or data on mortality associated with the inclusion of MUAC in the WaSt case-definition to make a strong recommendation for a change. (3) We could not explore other factors associated with WaSt due to lack of data beyond age and sex in our datasets. (4) To be consistent with the methodology employed to collect the data used in this study, we used SMART flagging criteria. They are based on statistical plausibility and exclude all values outside of ±3 Z-scores from surveyed population mean. This means that some cases that were potentially biologically plausible may have been removed.