Pitfalls in the somatomedin-C radioimmunoassay.

We have investigated the effects of various modifications on the quantitation of Somatomedin-C (Sm-C) by the Sm-C radio immunoassay. We have confirmed that the assay quantitatively measures Sm-C in serum at plasma samples when borosilicate tubes and protamine free buffers are used. Polystyrene tubes and/or protamine containing buffers interfere with the quantitative measurement of the hormone, apparently by modifying the binding of Sm-C to its binding protein. The implication of these modifications on the clinical use of the Sm-C RIA are discussed.