EFFECT OF TIME BETWEEN MEASUREMENTS ON WITHIN-SUBJECT VARIABILITY FORTOTAL CHOLESTEROL AND HIGH-DENSITY-LIPOPROTEIN CHOLESTEROL IN WOMEN

A single blood cholesterol measurement may not accurately reflect an i ndividual's true mean concentration. If duplicate blood samples are ta ken, what number of days between sampling gives the best chance of det ecting the maximum within-subject variation? In this study, we analyze d 20 serial blood samples obtained from each of 13 healthy, menstruati ng women over 35 days. Variability was calculated as the semivariogram , which gives the average squared difference between replicate samples taken over a range of sampling intervals. Data were available for a c omplete set of intervals from 1 to 26 days. Variability in total chole sterol (TC) increased as the interval between sampling increased from 1 to 12 days. With high-density lipoprotein cholesterol (HDL-C), varia bility increased from 1- to 7-day intervals. In practice, our results suggest that, irrespective of the time of menstruation, the minimal in terval for collecting a second blood sample for TC and HDL-C assays is similar to 2 weeks.