The Analytical Goals for Hemoglobin A1c Measurement in IFCC Units and National Glycohemoglobin Standardization Program Units Are Different

Abstract

To the Editor: The variation of a biological measurand can be expressed in the units of the measured concentrations or as a percentage of the absolute variation relative to the mean concentration. For example, given that different metrologic systems are in use for the measurement of human body temperature, this parameter can be expressed in degrees Celsius (Europe), degrees Fahrenheit (US), or degrees Kelvin (scientists). The equivalent unitary variation is 1.0 °C, 1.8 °F, or 1.0 K, respectively. Expressed as a percentage of the mean body temperature, this variation corresponds to 2.7% (1/37 × 100) for degrees Celsius, 1.8% (1.8/99 × 100) for degrees Fahrenheit, and 0.3% (1/310 × 100) for degrees Kelvin. From these results, one might conclude that temperature variation is lowest for scientists and highest for Europeans. Of course, that is nonsense. This wrong conclusion derives from the fact that variation across metrologic systems cannot merely be compared in terms of relative percentages when the y intercept (b) in the generic conversion equation ( y = a x + b) is not equal to zero. A higher y -intercept value will have a …