Understanding MUAC as a Screening Tool
Mid-upper arm circumference, or MUAC, is a simple, quick, and non-invasive measurement that requires minimal equipment—typically a non-stretchable measuring tape. Unlike body mass index (BMI), which requires accurate height and weight measurements that may be difficult to obtain for immobile patients or in resource-limited settings, MUAC can be easily performed by trained community health workers. The measurement primarily reflects the body's muscle mass and subcutaneous fat stores, which are sensitive indicators of nutritional depletion. For these reasons, MUAC has emerged as a valuable alternative for screening and identifying individuals at risk of malnutrition.
The Challenge of Defining a Universal Cutoff
While MUAC is well-established for screening severe acute malnutrition (SAM) in children, with internationally recognized cutoffs like <11.5 cm for SAM, a universally agreed-upon global cutoff for adults remains elusive. Research studies across different populations have yielded varying MUAC thresholds that correspond to the World Health Organization's (WHO) adult underweight classification (BMI < 18.5 kg/m²). These differences can be attributed to variations in ethnicity, sex, body composition, and overall health status among the study populations. For this reason, a single, definitive MUAC number for the underweight BMI category in cm does not exist, but a range of widely cited values provides useful guidance.
Specific MUAC Cutoffs from Global Studies
Meta-Analysis Findings (Global)
An individual participant data meta-analysis (IPDMA) aimed to determine a global MUAC cutoff for adults. The analysis of data from numerous studies across different continents found that MUAC cutoffs ranging from ≤23.5 to ≤25.0 cm could serve as an appropriate screening indicator for underweight. The study highlighted that a single cutoff of 24 cm was a reasonable starting point for screening in a community setting.
Regional and Population-Specific Studies
- Bangladesh (2015 study): A study focusing on Bangladeshi adults found sex-specific cutoffs to be more accurate. It recommended a MUAC of <25 cm for males and <24 cm for females as simpler alternatives corresponding to BMI < 18.5.
- Nepal (2020 study): Research conducted in Nepal suggested a MUAC cutoff of 24.5 cm as optimal for both men and non-pregnant women to identify underweight.
- Sudan (2022 study): A cross-sectional study among adults in eastern Sudan proposed a MUAC cutoff of ≤ 25.5 cm for both males and females to screen for underweight.
- Hospitalized Patients (2016 study): A study evaluating MUAC in hospitalized patients found that a value of ≤22.5 cm correlated well with a BMI < 18.5, independent of the patient's sex or age. This illustrates how context can influence the appropriate cutoff.
- Malnutrition Universal Screening Tool (MUST): The UK-based MUST assessment tool suggests that a MUAC of <23.5 cm indicates a likely BMI of less than 20 kg/m². It's important to note this is an estimation, not a precise correlation for BMI < 18.5.
Comparative Overview of MUAC Cutoffs for Underweight (BMI < 18.5)
| Study/Guidance | Population | MUAC Cutoff (CM) | Notes | References |
|---|---|---|---|---|
| IPDMA (Global) | Non-pregnant Adults | ≤23.5 to ≤25.0 (suggested 24 cm) | Range representing optimal balance between sensitivity and specificity. | |
| Bangladesh (2015) | Adults (Sex-specific) | <25 (Male) / <24 (Female) | Best fit for local population data, recommended for simplicity. | |
| Nepal (2020) | Chronically ill and healthy adults | 24.5 | Optimal cutoff found in both sexes in this specific study population. | |
| Sudan (2022) | Adults (Eastern Sudan) | ≤ 25.5 | Proposed as an alternative for community-based screening in that region. | |
| MUST (UK) | Adults | <23.5 | Suggests likely BMI < 20, but not a direct equivalent for BMI < 18.5. | |
| Inpatients (2016) | Hospitalized patients | ≤22.5 | Specific to a clinical inpatient setting, independent of sex and age. |
How to Measure MUAC Correctly
To ensure an accurate MUAC reading, standardized procedures must be followed.
- Preparation: Use a non-stretchable measuring tape. The patient should be relaxed, and the non-dominant arm should be bare and hanging loosely at their side.
- Locate Midpoint: Identify the bony protrusion on the shoulder (acromion process) and the tip of the elbow (olecranon process). Measure the distance between these two points with the arm bent at a 90-degree angle.
- Mark the Midpoint: Mark the halfway point on the arm. For example, if the measurement is 36 cm, the midpoint is at 18 cm.
- Take the Measurement: Ask the patient to let their arm hang loose again. Wrap the tape measure snugly around the arm at the marked midpoint, ensuring it is not too tight or loose. Record the measurement to the nearest millimeter.
Conclusion
The MUAC cutoff for the underweight BMI category in cm is not a single, universally accepted value for adults, but rather a range based on various study populations and contexts. While a global meta-analysis suggests a cutoff around 24 cm for general screening, region-specific studies indicate different thresholds, with sex often being a factor. The values typically fall between 23.5 cm and 25.5 cm to correlate with an adult BMI below 18.5. Ultimately, MUAC remains a highly valuable and practical screening tool, particularly in situations where measuring BMI is not feasible, provided the specific context and population are considered when interpreting the result.
For more detailed information on the correlation between MUAC and BMI, refer to the study published in BMC Public Health exploring MUAC as a substitute for BMI in adult nutritional assessment.
