Mid-Upper Arm Circumference Tapes and Measurement Discrepancies: Time to Standardize Product Specifications and Reporting

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FIGURE 1
Two Mid-Upper Arm Circumference Tapes. Top: Type A (thickness: 180 microns; width: 17 mm; measurement: the scale measures the outside circumference of the tape); Bottom: Type B (thickness: 280 microns; width: 25 mm; measurement: the scale measures the inside circumference of the tape)
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FIGURE 2
Measurements by Same Individual Using Same Object but Two Different Mid-Upper Arm Circumference Tapes. Left: Type A, 167 mm; Right: Type B, 165 mm

TABLE.

Systematic Bias (Built-in Error) in MUAC Tapes With Varying Thickness

Type A Tape (with material thickness of 180 microns)	Type B Tape (with material thickness of 280 microns)
Example 1: On a 100 mm true circumference, we have: D = C/π = 100/3.14159 = 31.8310
C = π × (31.8310 + 2^{^a} × 0.18) = 101.131 mm Built-in error of about 1.13 mm	C = π × (31.8310 + 2^{^a} × 0.28) = 101.759 mm Built-in error of about 1.76 mm
Example 2: If we consider a 200 mm circumference, we have: D = C/π = 200/3.14159 = 63.6620
C = π × (63.6620 + 2^{^a} × 0.18) = 201.131 mm Built-in error also 1.13 mm	C = π × (63.6620 + 2^{^a} × 0.28) = 201.759 mm Built-in error also 1.76 mm

Abbreviations: D, diameter; C, circumference; π (pi); MUAC, mid-upper arm circumference.
↵a Need to add twice the thickness of the material (For more detailed explanation of why the error is constant despite different circumferences, see the “String Girdling Earth” mathematical puzzle).