Thresholds for intervention using WHO standards

The thresholds approach is problematic since prevalence has different meanings in different contexts. A simple example is that 10% before harvest at the end of the "lean period" may not be too worrying but 10% a month after harvest is very worrying. Prevalence should always be interpreted in context. Context can be provided by (e.g.) the use of disease, agricultural, and food availability calendars; food-security / household economy data; and market monitoring.

There are additional problems when using W/H since this indicator (regardless of whether NCHS or WGS) is known to lack consistency between populations. Studies that have looked at body-shape and W/H indicate that W/H tends to underestimate the prevalence of acute undernutrition in Central American populations and overestimates prevalence in North African pastoralist populations. Overestimation is also probable in Indian and Myanmar populations. These findings, together with the well-described poor prognostic performance of W/H compared to all other anthropometric indices, suggest that MUAC is a better indicator.

There has been some work comparing prevalence by W/H using NCHS and WGS references. Much of this is available on the WWW. The principal finding of this work is that WGS prevalence tends to be higher than NCHS global prevalence and considerably higher SAM prevalence.

Should thresholds be revised? I think that this will not be useful unless guidance on interpretation WRT context is also given.

Is the WGS reference useful? I am not so sure WRT W/H.

Mike's message contains a lot of sense. I think that we need to be careful when using example comparisons such small camp vs. large free-living population. In the camp setting we almost always have a clear duty of care toward the population and a prevalence of (e.g.) 10% outside of reception camps or the initial establishment of camps is indicative of poor general ration distribution procedures, poor infection control, &c. These are things that we should be getting right in this context. This is a somewaht niggling point as Mike is correct in stating that these tresholds need to be abandoned and we should evaluate emergencies with regard to the magnitude of need and the ability of existing services to meet need. Looking at things this way is likley to improve targetting but may also lead to more sustainable programming in which supoort is given to existing services rather than a knee-jerk leap to parrallel and vertcial interventions.

Mike's comments on MUAC are, I think correct. Low MUAC is strongly predictive of near-term mortality and is treatable. I second Mike's obseravation that MUAC should not be corrected for height or age.

I am interested in what Mike says about the Prudhon Index. I am not very familiar with the use of this index. My understanding is that the Prudhon Index is an "audit" tool intended to create a standard against which a program can be compared. In this application, the investigator uses the index to create an expectation of mortality from characteristics of the program population and then compares the observed number of deaths to the expected number of deaths. If observed > expected then further investigation and remedial action is indicated. The index was developed as a way of predicting a number of deaths in a TFC patient cohort with an upper threshold at admisison of, I believe, 70% W/H median. This is about deaths IF ADMITTED not "death if (not) admitted". This is not to say that the index should not be used for prevalence estimation by survey. Such an index /might/ fix some of the weaknesses in the W/H case-definition and, given that calculating WHZ in a survey dataset is computerised the calculations involved are no argument against its use and an alternative to WHZ. I do think, however, that examination and possible recalibration of the index on a wider population would be required before its use in this role. Suitable data should be available from the many previous cohort studies examining the relationship between anthropometry and mortality.

One concern that I have with the Prudon Index is that it is basically a sum of a modified BMI (i.e. the log of weight divided by height raised to some power) and oedema with some constants thrown in passed through a logistic linking function (to yield a value between 0 and 1 as a probability of near-term death). This suggests, to me, that it will suffer a body-shape bias and overestimate probabilities for log-legged / short-trunked individuals.

Sorry. I misunderstood your intented meaning. There are validated formulae for calculating BSA from weight alone which have been developed for drug-dosage calculation. I have recently done some work on this for the WHO for ARv drug-dose calculation and have a draft paper prepared if you'd like to see it. As for body shape, MUAC, and W/H ... I did a study for ENCU / UNICEF which was published in last months Annals of Human Biology. This found that W/H is strongly associated with body shape (SSR) and MUAC only weakly associated with body shape (and not at all if case-status such as MUAC < 125 mm is used).