Coefficient of variation is commonly used as a means of measuring the variability of an instrument. The data are gathered by recording the values for the normal and abnormal controls for each test run. At the end of the month, the standard deviation, mean, and coefficient of variation are calculated. The testing data for a particular instrument might look like this:
| Normal Control | Abnormal Control |
|  | s | CV | | s | CV |
| January | 100.9 | 2.43 | 2.41 | 209.5 | 4.41 | 2.11 |
| February | 103.1 | 2.99 | 2.90 | 211.6 | 4.00 | 1.89 |
| March | 102.0 | 2.21 | 2.17 | 206.8 | 3.95 | 1.91 |
The coefficient of variation stays fairly constant from month to month. If there is a sudden increase, there might be a problem with the method or the equipment.
In the clinical laboratory, the use of CV as a measure of relative variability should not be confused with the use of the standard deviation as a measure of absolute variability. For example, support physicians agreed that for accurate patient treatment, the inherent variability in a glucose method should be less than 5 mg/dL. In this case, neither the hexokinase nor the orthotoluidine method is acceptable. It does not matter which is more precise if neither is precise enough to result in adequate patient care.
