Why Measure Mid-Upper Arm Circumference (MUAC)?
Mid-upper arm circumference (MUAC) is a simple, non-invasive, and effective tool used in both clinical and field settings to assess nutritional status in adults. Unlike Body Mass Index (BMI), which requires both height and weight measurements, MUAC can be taken with minimal equipment and training. It is particularly useful when height and weight are difficult to obtain, such as with bedridden patients or in remote areas. MUAC measurements primarily reflect changes in muscle and subcutaneous fat, making them a sensitive indicator for assessing undernutrition and, more recently, overweight and obesity in adult populations. A strong correlation between MUAC and BMI has been observed across various adult populations, demonstrating its reliability as a proxy for nutritional status.
The Step-by-Step Procedure to Measure MUAC in Adults
This guide outlines the standardized procedure for accurately measuring MUAC in an adult. For consistency, it is generally recommended to measure the non-dominant arm, though either arm can be used if necessary.
Step 1: Prepare Your Equipment and Subject
- Equipment: You will need a flexible, non-stretchable measuring tape. Ideally, an anthropometric or MUAC-specific tape is used, but a standard sewing tape measure will work if it does not stretch. A pen or marker is also useful for marking the midpoint.
- Positioning: Ask the person to bend their non-dominant arm at a 90-degree angle at the elbow, with the upper arm parallel to their side. The forearm should be relaxed and held in front of them.
- Clothing: It is best to take the measurement directly on bare skin. If clothing is necessary, it should be a very thin layer and not compress the arm.
Step 2: Locate and Mark the Midpoint
- Landmarks: Locate two key anatomical landmarks: the tip of the shoulder (the bony protrusion called the acromion process) and the tip of the elbow (the olecranon process).
- Measure Length: Run the measuring tape from the acromion process down to the olecranon process.
- Find Midpoint: Identify the exact midpoint of this length. For example, if the total length is 32 cm, the midpoint is at 16 cm. Mark this midpoint with a pen.
Step 3: Take the Circumference Measurement
- Relax the Arm: Instruct the person to relax their arm and let it hang loosely by their side. It is crucial that the arm is not flexed or tensed, as this will distort the reading.
- Wrap the Tape: Wrap the measuring tape around the arm at the marked midpoint.
- Apply Correct Tension: Ensure the tape is snug against the skin but not so tight that it indents the flesh or too loose that it can be easily moved.
- Read the Measurement: Read the measurement to the nearest millimeter (or 0.1 cm) where the tape overlaps. It is a best practice to take three separate measurements and record the average for greater accuracy.
Potential Sources of Error
Several factors can lead to inaccurate MUAC measurements. Avoiding these common mistakes is essential for obtaining reliable data.
- Incorrect Midpoint: Measuring above or below the true midpoint will result in an incorrect circumference.
- Improper Tension: The tape must be taut but not constrictive. Pulling the tape too tight will give a falsely low reading, while a loose tape will yield a falsely high one.
- Flexed Muscles: A person flexing their arm muscles during the circumference measurement will produce an inflated reading. Always ensure the arm is fully relaxed.
- Using a Stretchable Tape: Cloth or other stretchable tape measures can give inconsistent results. A non-stretchable material is required for accuracy.
Comparison of MUAC and BMI
While both MUAC and BMI assess nutritional status, they differ in their application, resource requirements, and limitations. Below is a comparison table outlining their key differences.
| Feature | Mid-Upper Arm Circumference (MUAC) | Body Mass Index (BMI) |
|---|---|---|
| Equipment | Non-stretchable tape measure | Weighing scale, stadiometer for height |
| Ease of Measurement | Simple, fast, and requires minimal training | Can be logistically challenging for immobile patients |
| Portability | Highly portable, ideal for field use | Equipment can be cumbersome to transport |
| Reflects | Muscle mass and subcutaneous fat | Body weight in relation to height |
| Limitations | Does not differentiate between fat and muscle mass | Not a reliable measure of body composition |
Nutritional Assessment and MUAC Cut-offs
For adults and adolescents aged 15-17, standardized MUAC cut-offs are used to screen for different nutritional states. These cut-offs can vary slightly depending on the organization or population studied, but generally follow similar patterns.
- Normal Nutritional Status: MUAC ≥ 220 mm (or 22 cm).
- Moderate Acute Malnutrition (MAM): MUAC ≥ 185 to < 220 mm (or 18.5 to < 22 cm).
- Severe Acute Malnutrition (SAM): MUAC < 185 mm (or < 18.5 cm).
It is important to note that these are screening tools and not definitive diagnoses. Clinical assessment is always necessary to confirm a patient's nutritional status. For reference, a MUAC of 23.5 cm or less in men may indicate poor nutritional status, while for women, the value is 22.0 cm or less, according to some studies.
Conclusion
Measuring mid-upper arm circumference is a highly practical and reliable method for assessing the nutritional status of adults, particularly in situations where traditional methods like BMI calculation are not feasible. By following a simple, standardized, step-by-step procedure and paying careful attention to proper technique, accurate and reproducible measurements can be achieved. This tool serves as a vital component in global health initiatives and clinical practice for the early detection and management of malnutrition, helping to ensure that individuals receive the appropriate care and nutritional support they need. While valuable, MUAC is just one piece of the nutritional assessment puzzle and should be used in conjunction with other clinical evaluations. For more information on health and nutrition guidelines, refer to organizations like the World Health Organization (WHO).
