Plasma concentrations of histamine measured by radioenzymatic assay: effects of histaminase incubations.

To help resolve the current uncertainty as to whether assays for plasma histamine are measuring non-histamine compounds as well, we compared the effects of prior incubation with histaminase and buffer on measurements in (1) normal plasma, (2) buffer and normal plasma to which several amounts of exogenous histamine had been added, and (3) plasma obtained after inhalation-induced asthma or form the site of a local heat urticaria challenge. As measured by the radioenzymatic technique, low (1 to 4 ng/ml) levels of histamine-like material were present in normal plasma after incubation with either histaminase or buffer. In contrast, histaminase (but not buffer) incubation markedly reduced measured histamine in all other specimens. Exogenous histamine in buffer was reduced almost 100% by histaminase, whereas the degree of reduction in plasma specimens varied directly with the starting histamine level. Therefore it appears that low levels of histaminase-resistant material reacting in histamine assays is present in normal plasma. The use of histaminase incubation appears to be helpful in differentiating this from true histamine release in allergic reactions.