Heparinized plasma cannot be used in which two assays?

• A. Lithium and Free drug assay
• B. Glucose and Hemoglobin A1c
• C. Creatinine clearance and Uric acid
• D. Cholesterol and Triglycerides

Answer: (A)

The key idea is that the anticoagulant in plasma can alter the sample in ways that interfere with some assays, especially those relying on precise ionic conditions or protein binding. Lithium determination uses an ion-selective electrode, and the ionic environment in heparinized plasma can change the electrode response, leading to inaccurate results. Therefore lithium is typically measured in serum or in plasma without interfering anticoagulants rather than in heparinized plasma. Free drug assays require measuring the unbound fraction of a drug, which depends on protein binding and the sample’s matrix; heparin can disrupt protein binding or the separation methods used to isolate the free drug, giving unreliable unbound concentrations. Hence, these two assays are not suitable for heparinized plasma. Other routine measurements like glucose, cholesterol, triglycerides, and uric acid are generally compatible with heparinized plasma.