Dear Ilaria,

You could use MUAC in children less than 6 months to identify those who are at high risk, but you should use a different cut-off. You could also use weight for age. All this is explained in the document available at:

https://www.ennonline.net/mamicasedetectionreview

Measuring MUAC for infants under 6 months is not valuable as it can not give us accurate results to assess the degree of child malnutrition. Therefore, MUAC is not dependable and not specific for such an age. 

On the issue of anthro indicators for infants <6m, do also check this systematic review

https://journals.sagepub.com/doi/full/10.1177/11795565211049904

Abstract

Background:

There is increasing global focus on small and nutritionally at-risk infants aged <6 months (<6 m). Current WHO guidelines recommend weight-for-length z-score (WLZ) for enrolment to malnutrition treatment programmes but acknowledge a weak evidence-base. This review aims to inform future guidelines by examining which anthropometric criteria best identify infants <6 m at high risk of mortality/morbidity.

Methods:

We searched Medline, EMBASE, CINAHL, Global Health, Cochrane Library and POPLINE for studies conducted in low- and middle-income countries and published between 1990 and October 2020. We included studies reporting anthropometric assessment of nutritional status in infants <6 m and assessed the association with subsequent morbidity or mortality.

Results:

A total of 19 studies were included in the final review, covering 20 countries, predominantly in sub-Saharan Africa. WLZ had poor reliability and poor prognostic ability to identify infants at risk of death. Mid-upper arm circumference (MUAC) and weight-for-age z-score (WAZ) were better at identifying infants at risk of mortality/morbidity. MUAC-for-age z-score did not perform better than using a single MUAC cut-off. Suggested MUAC cut-offs for this age group varied by context, ranging from 10.5 to 11.5 cm. The assessment for reliability showed that length was difficult to measure, making WLZ the least reliable indicator overall.

Conclusion:

Evidence from our review suggests that a change in current practice is necessary. To better identify small and nutritionally at-risk infants <6 m WAZ and/or MUAC rather than WLZ should be used. Future research should explore possible benefits for programme coverage, impact and cost-effectiveness. Research should also examine if context-specific MUAC thresholds are needed.

There is still a growing evidence on MUAC to be used to identify nutrition vulnerablilty  in infants under 6 months.However,classification cut-off have not been established for children less than 6 months.