Effect of Different Sample Types and Stability after Blood Collection of N- Terminal Pro-B-Type Natriuretic Peptide as Measured with Roche Elecsys System To the Editor: N-Terminal pro-B-type natriuretic peptide (NT-proBNP; 76-amino acid

an extraction and a purification step, requires validation by a definitive or reference method before being considered a gold standard (2 ). We have not made an arbitrary statement that extraction/chromatography RIAs for testosterone do not work. Our point is that there are no convincing data showing that they do work. The only existing reports, from two independent sources (1, 5), that compared extraction/ chromatography RIA with gas chromatography–mass spectrometry show that there are considerable problems with these analyses. Until an extraction/chromatography RIA has been properly validated the results generated from such tests are subject to question. We appreciate the opportunity to further emphasize the need for proper validation of steroid immunoassays.