Equivalence between international units and mass units of alpha-foetoprotein. Report of a collaborative study.

A study aimed at establishing a correspondence between the international unitage which presently defines the activity of the WHO alpha-foetoprotein (AFP) standard and mass units of AFP was carried out in collaboration with 11 laboratories from seven countries. Various methods of assay, including radial immunodiffusion, enzymo-immunoassay, Laurell rocket immunoelectrophoresis, radioimmunoassay and radio-rocket immunoelectrophoresis, were used. The mean value of 11 individual estimates indicated that one international unit (I.U.) was worth 1.17 ng of AFP (90% confidence interval = 1.06-1.28). Since there was no objective basis on which to evaluate the relative purity of AFPs from various sources, it was felt that each purified AFP should be given equal weight; accordingly when only results from the six laboratories which purified their reference material were taken into account, one I.U. was found to be worth on average 1.21 ng of AFP (90% confidence interval = 1.02-1.43). It is proposed that the official correspondence between international and mass units be based upon that equivalence. It is interesting to compare these results with those of the first collaborative study in 1974; at that time, one I.U. was found to be worth 1.19 ng of AFP on average which is very close to the 1.21 ng figure. The 90% confidence interval was 0.93-1.52. Although there is better agreement between laboratories in the second study, the distribution of individual equivalences is still quite heterogenous, and it is therefore recommended that the present international unitage continue to be used.