Intraperson variability. Magnitude, source, and influence on interpretation and diagnosis.

In a person in a steady state the set of values obtained for a test carried out on a series of samples will show fluctuation about a value (the setting). In practice a single test value is usually taken as an estimate of the setting. The general consequences of this approximation are discussed in relation to the various uses of data in diagnosis, and illustrated for the common measurements of clinical chemistry. The magnitude of these consequences depends on the proportion of the total variation in a group of persons which is caused by these fluctuations in the person--within-person variation (VarWP). When the proportion is high the aim should be to reduce VarWP. When VarWP is high because of analytical error, then improved analytical technique or replicate analysis is required. Otherwise standardised techniques and conditions for venepuncture or, as a last resort, repeated samplings are necessary. These problems are discussed in relation to the detection of hypokalaemia.