Potassium (K) in Serum. Potassium analysis in serum is a laboratory test that measures the level of potassium in the blood, a vital mineral that plays a key role in many bodily functions. This test is used to assess fluid balance, blood pressure, and muscle performance, including the heart. It is recommended for individuals experiencing symptoms such as muscle weakness, confusion, heart rhythm disturbances, or those taking medications that may affect potassium levels. Normal potassium levels typically range between 3.5 and 5.0 mg/dL. Abnormal levels may indicate health issues that require further evaluation and appropriate treatment.