Radioassay of serum folate, with use of a serum blank and nondialyzed milk as folate binder.

Serum folate determination by this method requires incubation of 0.1 ml of serum (or standards) with nondialyzed milk and then with tritium-labeled folate. A serum blank corrects for nonspecific binders present in the serum. Unbound folate is removed with hemoglobin-coated charcoal. The standard deviation of the results is ±G.2 pig/liter, and more than 90% of folate added to the serum is accounted for analytically. Serum folate concentrations as determined by this method agree with those obtained by microbiological assay (Lactobacillus casei), and our method is faster, simpler, and more accurate than other radioassay methods.