Between and within subject variation of the first phase insulin response to intravenous glucose.

Eight normal subjects underwent two intravenous glucose tolerance tests to determine the between and within subject variation of the first phase insulin response. Variability was represented by the coefficient of variation. The between subject variation for the incremental 0-10 min insulin area was 58%, and the within subject variation was 22% (median value), range 3-55%. The variation of the first phase response expressed in four different ways was compared. The total and incremental (above fasting levels) 0-10 min areas provided less variable results (variation 52 and 58%) than the 1 + 3 min insulin levels (variation 72%) or mean of the incremental 3-5 min insulin levels (variation 66%). The ratio of the incremental 0-10 min insulin to glucose areas was as variable (variation 53%) as the insulin responses alone. The variability of insulin responses to intravenous glucose severely limits their value as early predictors of B-cell failure.