Coupon Codes not valid for this testing. Fasting Required: No Lab: Quest Diagnostics Specimen: Blood Results: Average processing time 3-5 days Note: Result turnaround times are an estimate and are not guaranteed. Our reference lab may need additional time due to weather, holidays, confirmation/repeat testing, or equipment maintenance. Description: The Potassium RBC test measures the Potassium level in the Red Blood Cells (RBCs). Potassium is important for nerve, muscle and heart functions. As an intracellular measurement of Potassium, the Potassium RBC test is more reflective of your longer term Potassium status.