It is common to say, as Woody does above, that MUAC is not valid for younger children but recent work by KEMRI and LSHTM shows that MUAC is, as it is in older children, strongly predictive of mortality and suggests lower MUAC thresholds (i.e. MUAC < 110 mm @ 6 to 14 weeks) that can be applied at routine vaccination ages and at birth (i.e. MUAC < 90 mm @ birth). See:
Mwangome, MK, Fegan G, Fulford T, Prentice AM, Berkley JA, MUAC at age of routine infant vaccination to identify infants at elevated risk of death: a retrospective cohort study in the Gambia. Bulletin of the World Health Organization 90, 887–894 (2013).
Mwangome MK, Fegan G, Mbunya R, Prentice AM, Berkley JA, Reliability and accuracy of anthropometry performed by community health workers among infants under 6 months in rural Kenya. Tropical Medicine & International Health 17, 622–629 (2012).
Mwangome M et al., Anthropometry at birth and at age of routine vaccination to predict mortality in the first year of life: A birth cohort study in BukinaFaso. PloS one 14, e0213523 (2019).
But ... "Research on appropriate clinical guidelines for the treatment of severe acute malnutrition in infants aged less than 6 months is needed to support effective interventions.".
Work by the MAMI special interest group has moved forward with identifying appropriate care pathways for these chidlren. That SIG favours the use of the weight-for-age z-score in children ages between 0 and 6 months. See:
Lelijvelds N, Kerac M, McGrath M, Mwangome M, Berkley JA, A review of methods to detect cases of severely malnourished infants less than 6 months for their admission into therapeutic care. ENN 1–14 (2017).
This article evaluates the performance of a number of anthropometric case-definitions for use in younger children.
I hope this is of use to someone.