Understanding the Mid-Upper Arm Circumference (MUAC)
Mid-Upper Arm Circumference (MUAC) is an anthropometric measurement that gauges the circumference of a person’s upper arm at its midpoint. This measurement is a good indicator of lean body mass and fat reserves, making it a valuable tool for nutritional assessment. Unlike BMI, which requires height and weight measurements that can be difficult to obtain for immobile or ill patients, MUAC can be taken easily and quickly with a simple, non-stretchable tape. This makes it particularly useful in emergency situations, mobile clinics, and for screening large populations in low-resource areas.
Why MUAC Cut-offs are not Universal for Adults
Unlike the relatively standardized MUAC cut-offs for children, establishing a single universal threshold for adults is challenging for several reasons:
- Population Diversity: Nutritional status can vary significantly across different populations due to genetics, lifestyle, and environment. A cut-off that works for one demographic may not be accurate for another.
- Aging: Body composition changes with age, which can affect MUAC measurements. Some studies suggest lower cut-offs for older adults compared to younger adults.
- Chronic Illness: Conditions like HIV or cancer can significantly impact nutritional status and body composition, making a single, general cut-off less effective.
- Purpose of Screening: The optimal cut-off depends on the objective of the screening. For instance, in emergency settings where the goal is high sensitivity to capture all potential cases, a lower cut-off might be used compared to a clinical setting aiming for higher specificity.
Key MUAC Cut-offs and Interpretations for Adults
Despite the lack of a single universal standard, several guidelines and studies provide important MUAC cut-offs for assessing adult nutritional status. These are often benchmarked against the WHO's BMI classifications.
Comparison Table of MUAC Cut-offs for Adults
| Nutritional Status | Correlating BMI (kg/m²) | General MUAC Cut-off (cm) | Contextual Variations & Considerations |
|---|---|---|---|
| Underweight/Chronic Energy Deficiency | BMI < 18.5 | ≤ 25.5 cm (Sudan Study) ≤ 24.5 cm (Nepal Study) |
Studies show slight variations. A large meta-analysis suggests a range of ≤23.5 to ≤25.0 cm is appropriate for identifying underweight adults. |
| Overweight | BMI ≥ 25.0 | 25.4 cm to < 29.9 cm (Ugandan Study) | A Bangladesh study suggests ≥28.5 cm for younger males and ≥27.5 cm for females for overweight detection. |
| Obesity | BMI ≥ 30.0 | ≥ 29.5 cm (Sudan Study) ≥ 31.1 cm (Ugandan Study) |
Cut-offs for obesity also vary. A Bangladesh study suggests ≥29.5 cm for younger males and ≥28.5 cm for younger females. |
| Acute Malnutrition (Emergency) | Various | < 21 cm (Severe) < 23 cm (Moderate) |
Humanitarian agencies often use these cut-offs, particularly for women of reproductive age, in emergency contexts. |
How to Measure MUAC Correctly
To ensure accuracy, the MUAC should be measured following a standard procedure:
- Measure the distance between the bony tip of the shoulder (acromion process) and the elbow (olecranon process) on the non-dominant arm.
- Mark the midpoint of this distance.
- Wrap a non-stretchable measuring tape firmly around the arm at this midpoint.
- The reading should be taken to the nearest millimeter while the arm is relaxed and hanging by the side.
Implications of MUAC Assessments in Public Health
The use of MUAC is an indispensable tool in public health and humanitarian efforts for its speed and practicality. In settings where access to medical equipment is limited, MUAC allows for rapid screening and classification of nutritional status, enabling timely intervention. For instance, aid organizations in emergency-affected populations use specific MUAC cut-offs to determine eligibility for supplementary or therapeutic feeding programs. For example, MSF has historically used cut-offs like MUAC < 210mm or < 230mm for admission criteria for pregnant and lactating women. However, as highlighted by a UNICEF toolkit, the choice of cut-off depends on resource availability and caseload, sometimes favoring a more conservative approach.
The Importance of Context
It's critical to interpret MUAC results within their specific context. An individual's MUAC reading is just one piece of the puzzle. Factors such as a person’s medical history, recent weight loss, and the presence of oedema should also be considered. In clinical settings, a MUAC assessment might prompt further evaluation with BMI or other, more comprehensive methods. For example, a MUAC of < 23.5 cm may indicate a likely BMI of < 20, but it does not replace a definitive BMI calculation. This tiered approach ensures resources are used effectively while identifying those most at risk.
Conclusion
While a single, globally accepted cut-off for the MUAC for adults does not exist due to variations in population, age, and context, various localized studies and humanitarian protocols provide actionable thresholds. The determination of the cut off for the MUAC for adults is a nuanced process. A large individual participant data meta-analysis has suggested a range of ≤23.5 to ≤25.0 cm as a strong screening indicator for underweight in non-pregnant adults. However, specific operational cut-offs for emergency situations (e.g., <21cm or <23cm) and population-specific thresholds are also widely used. For accurate assessment, the MUAC should be measured correctly and interpreted alongside other clinical information, especially in resource-constrained environments where it remains an invaluable and practical tool for identifying malnutrition.
For more information on nutritional screening tools, consider visiting the Malnutrition Universal Screening Tool (MUST) resource page for guidelines often used in the UK.
