Statistical pitfalls in the comparison between two commercial serum ferritin kits, Pharmacia and Amersham.

Two commercial ferritin kits, Phadebas Ferritin PRIST (kit B) and Ferritin RIA Amersham (kit A) were compared in order to "translate" ferritin values from one kit to the other. Ferritin levels in 222 sera were determined with both kits in the concentration range 5-838 micrograms/l. Regression analysis disclosed a parabolic regression between the logarithmically transformed results obtained with the two kits. Measured kit B values of 10, 12, 15, 30 and 300 micrograms/l corresponded to calculated kit A values of 15, 17, 20, 34 and 321 micrograms/l, respectively. In kit A, storage of sera for two years at -25 degrees C in combination with freeze-thawing three times produced a minor fall (p < 0.0001) in ferritin levels with a median percent decline of 16%.