Which of the following assays uses indirect measurement using a chromogenic substrate to quantitate heparin?

Question

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The heparin anti-Xa assay is used to indirectly measure the concentration of heparin.In this system,heparin has an inhibitory effect on the enzyme Xa.Xa normally proteolytically cleaves the chromogenic substrate,releasing the chromogen with a color intensity that is then measured.Because heparin has inhibited some of the Xa,less free Xa will be available to cleave the substrate and less color will be released.Thus the concentration of heparin is inversely proportional to the activity of Xa.This is classified as an indirect measurement.

Which of the Following Assays Uses Indirect Measurement Using a Chromogenic