All tests are subject to analytical variation as well as biological variation. The uncertainty of measurement (UOM) figure indicates the analytical (laboratory method) imprecision. The UOM is defined as twice the %CV (coefficient of variation), determined by repeated testing of control samples over a long period (typically 6 months).

Uncertainty of measurement can be used in 2 ways:

For a single test result:

There is a 95% probability that the true result will lie between quoted result + UOM and quoted result - UOM.

When comparing 2 test results:

If the difference between two test results is greater than the uncertainty of measurement, there is an 84% probability that the difference represents a real change (i.e. the difference is not due to method imprecision alone). This change could be due to normal biological variation or a change in the patient's condition.

Some uncertainty of measurement figures are quoted in absolute units at specified concentrations. Where the UOM. is given as a percentage, it applies to concentrations close to the reference interval unless otherwise specified.