Why prevalence using MUAC is not useful as trigger level for humaniterian response?

Big and important questions!

One of the problems with W/H based indices (including BMI) is that they are strongly influenced by body-shape as measured by the sitting to standing height ratio (SSR) or similar measures such as the ratio of trunk to limb length.

Pastoralists in warm climates and at low altitudes are known to have low SSR (i.e. short trunks and long limbs). Low SSR leads to lower means W/H and higher prevalence of GAM in pastoralist populations.

Since SSR varies widely between ethnic groups and locations the relationship between W/H and mortality will also vary widely between ethnic groups and locations (this has been observed and reported in the scientific literature). The relationship between MUAC and mortality does not vary much between ethnic groups and locations (this has been observed and reported in the scientific literature). Incidentally, at the other end of the distribution we have mountain dwellers who tend to have short limbs and broad chest - W/H makes these populations appear "overweight" or "obese".

I used "lower" and "higher" above because to use "underestimate" and "overestimate" requires a "gold standard" of nutritional status. It is usual to consider W/H as the "gold standard" but there is no evidence to support this practise - It is just a nasty habit we seemed to have picked up some time in the 1980s.

The terms "nutritional status" and "anthropometric status" are often used interchangeably. Nutritional status refers to the internal state of an individual as it relates to the availability and utilisation of nutrients at the cellular level. This state cannot be observed directly so observable indicators are used instead. There are a range of observable indicators (biochemical, clinical, and anthropometric) of nutritional status, none of which taken alone or in combination are capable of providing a full picture of an individual's nutritional status. There is, therefore, no single "gold-standard" indicator of nutritional status.

Nutritional status can be usefully defined at the individual, as opposed to the cellular, level as the ratio of nutrient reserves (muscle and fat) to the nutrient requirements of organs (brain, liver, heart, kidneys, lungs, &c.). It is generally recognised that muscle plays a special role as a nutrient reserve during infection and that infection is a major etiological factor in acute undernutrition. W/H expresses the relationship between weight and height. In children, about 4% of weight is nutrient reserves in muscle. About 96% of weight is, therefore, unrelated to nutrient reserves. Height is almost completely unrelated to the nutrient requirements of organs. W/H measures, therefore, the ratio of something that does not strongly reflect nutrient reserves to something that is almost completely unrelated to nutrient requirements. MUAC, however, is directly related to muscle mass and is, therefore, a direct measure of nutrient reserves.

The limited evidence that is currently available suggests that an index known as the lean-mass ratio (LMR), the ratio of the estimated mass of the limbs to the estimated mass of the trunk, is the best anthropometric indicator of nutritional status. The available evidence suggests that MUAC uncorrected for age or height is a better indicator of nutritional status than all other practical indicators and that W/H is not associated with LMR and is the worst practical indicator of nutritional status.

An alternative to examining the association between an anthropometric indicator and nutritional status is to examine the prognostic or predictive value (i.e. of predicting death) of various indicators. This approach makes a great deal of sense if you consider that we are running child survival programs. When this has been done, W/H has been consistently shown to be least effective predictor of mortality and that, at high specificities, MUAC is superior to both height-for-age and weight-for-age.

In terms of indicators that are practical to collect in developmental and emergency settings, MUAC has the best claim to be a practicable "gold standard" of nutritional status.

If you accept these arguments then the "gold standard" to use is MUAC. This means that in all populations (not just in pastoralist populations) MUAC should be the preferred indicator. Now we have decided on a "gold standard" we can see that W/H overestimates prevalence in pastoralist populations. NOTE : W/H will underestimate prevalence in (e.g.) mountain dwellers and cold climate populations.

Now for something controversial ... The question of thresholds is interesting. My view is that the thresholds are about GAM and were established for GAM. With MUAC we measure GAM but with W/H we mistakenly measure something else or something confused / confounded. With MUAC we have a more useful indicator for GAM and, I think, we can use the existing thresholds.

To answer the big question in the title of this thread "Why prevalence using MUAC is not useful as trigger level for humanitarian response?" is difficult. I think that MUAC is the most useful indicator. I think the real questions should "How on Earth did we ever decide that W/H should be used for this purpose?" and "How can we stop W/H being used?".

These (i.e. 5%, 10%, 15%) are nice round numbers. I do not think it was ever intended that they be linked to specific indicators (e.g. they have not been changed by the ongoing shift from NCHS to WGS references) and meant to refer prevalence of a morbid condition known as "acute malnutrition". I am away from my office at the moment and can't shuffle through my old library boxes of articles but I think you might find that the same thresholds were used when we used W/A, MUAC, and MUAC/H case-definitions in the pre-W/H era. I will be in the field for another month or so ...so ... can anyone check this out?

I think that the same thresholds can be used for GAM prevalence by MUAC using the MUAC < 125 mm case-definition.

I would be very interested to hear what others think about this issue.

I am not convinced that "everyone is addicted to the WHO thresholds. Most of the decision makings are influenced by these magic threshold figures".

I am writing this from Bangladesh. Reputable surveys (e.g. DHS) from here have the prevalence of wasting (defined as WHZ < -2) as being consistently above 20% but we don't talk every year about the famine in Bangladesh. In Somali Region of Ethiopia (e.g.) prevalence (again by WHZ < -2) is regularly reported as being above 20% but we don't have DEC appeals and start airlifting in food every time that happens. I think that these thresholds are very often ignored.

At the other end of the thresholds ... In the southern African famine of 2002 / 2003 (this is the one where Zambia, a supposedly famine-afflicted nation, refused GM maize) we had a full-blown regional intervention on the grounds of a handful of dubious quality household economy assessments and vulnerability assessments from a small number of settings. I was working with VALID doing CTC development work at this time in Malawi during a "national nutritional emergency" and we saw wasting prevalence (again by WHZ < -2) rocket to an astonishing 4.5% in one district. I was also working for ITI developing a survey method for trachoma in Malawi in 2002 and an INGO working in the district found wasting to be as high as 6%. I had my teams screen dozens of villages in the same district using MUAC and we found a handful of cases.

There must be reasons for this. There are "aggravating factors" and there are "political factors". I think there is also the problem that as long as we continue to use W/H we have a prevalence estimator that has no clear or consistent meaning.