Abstract
Immunoassay methods for prolactin detect macroprolactin (i.e. high molecular mass complexes of prolactin) to various degrees. Therefore it is generally assumed that the widely differing results by methods that measure both moieties to a differing extent are due to the presence of macroprolactin. We present a case which challenges such an assumption and suggest that precipitation by polyethylene glycol is the most reliable screen for identifying macroprolactin (and/or interfering antibodies if present).