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.