When the chromogenic assay for antithrombin is performed, which of the following is true?
a. The colored end-product intensity is inversely proportional to the antithrombin concentration.
b. The chromogenic substrate is bound by antithrombin.
c. Warfarin is added to the patient sample before testing to improve sensitivity.
d. Activated protein C competes for the chromogenic substrate, so it must first be selectively removed.
ANS: A
Antithrombin concentration is inversely proportional to the intensity of end-product color. Heparin is added to the patient plasma before testing to improve sensitivity to antithrombin. A protease, often Xa, is added in excess to an aliquot of the patient sample. Some of this Xa will be bound by the heparin-antithrombin complex and thus inhibited. After this, a chromogenic substrate is added that is specific for Xa. When the residual Xa hydrolyzes this substrate, color will be released. The more antithrombin present in the patient sample, the more Xa will be bound and the less chromogen will be released.
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