The technology described in this document can be embodied in a test strip for use in measuring a level of an ST2 cardiac biomarker in a whole blood sample. The test strip includes a base, and a plurality of conjugates, wherein each conjugate includes a reporter group bound to a first antibody that binds to ST2. A conjugate pad holds the plurality of conjugates that bind with ST2 to produce conjugate-ST2 complexes. The test strip also includes a plurality of second antibodies that bind to ST2, and a plurality of third antibodies that bind to the conjugate-ST2 complexes. The plurality of second antibodies are bound to a membrane in a test location, and the plurality of third antibodies are bound to the membrane in a control location. A plasma separation pad passes blood plasma from a whole blood sample to the conjugate pad while inhibiting other components.