In a patient with a myeloproliferative disorder, serum potassium may be affected by platelet release. For accurate potassium, which sample should be tested?

• A. Repeat testing of the original serum
• B. Testing freshly drawn serum
• C. Testing heparinized plasma
• D. Atomic absorption spectrometry

Answer: (C)

Platelets release potassium when blood clots, so potassium measured in serum (the product of clotting) can be falsely elevated, especially when platelet counts are high as in myeloproliferative disorders. Using a sample that is prevented from clotting—heparinized plasma—stops this platelet release, giving a value that reflects the true circulating potassium rather than an artifact from clot formation. Repeating on the original serum won’t fix the artifact, and freshly drawn serum would still be serum after clotting. The choice of atomic absorption spectrometry is about the method, not the sample type, so it doesn’t address the clotting artifact.