Intra-individual variation: significant changes in parameters of lipid and carbohydrate metabolism in the individual and intra-individual variation in different test populations.

Fasting serum triglyceride, cholesterol, HDL cholesterol, HDL 2&3 cholesterol, fasting plasma glucose and insulin and haemoglobin A1 were measured under standardised conditions in a group of laboratory volunteers. Intra-individual variation was calculated for each parameter from weekly measurements on at least eleven successive occasions, and the minimum change in each parameter that would be significant at the level P less than 0.05 was calculated. A further study compared intra-individual variation in different test populations. Data from the group of laboratory volunteers were taken to represent intra-individual variation under standard test conditions with an informed test population and data from a group of regularly monitored out-patients undergoing drug therapy for hypercholesterolaemia were taken to represent intra-individual variation under standard conditions in a typical test population. A group of in-patients under strict dietary control provided information on variation under extreme standardisation of test conditions. Intra-individual variation was greatest for all parameters in the outpatient population. Extreme standardisation of test condition reduced intra-individual variation beyond that of the laboratory volunteers only in the case of serum triglyceride.