Interlaboratory comparison of serum thyroxine analyses

Approximately 300 clinical chemistry laboratories participated in a survey of measurements of the thyroxine (T 4 ) concentration in 13 lyophilized serum specimens prepared by spiking a base serum pool with several different levels of T 4 . Of 35 commercially available kit methods, 4 kits were used by 30 or more laboratories, and 9 by 10 or more laboratories. Values obtained with the Abbott or Nuclear Medical laboratories, kits, which were used by one-third of all laboratories that named a specific kit, averaged 1 to 2 ug/dL greater than those obtained with the other methods. Within-run coefficients of variation were 10 to 20% for specimens containing less than 3 ug/dL of T 4 and 3 to 6% for specimens with more elevated T 4 levels.