A questionnaire to assess knowledge of the expected elevation in serum K measurement with different grades of hemolysis was administered to medical technologists working in biochemistry laboratories, hospital physicians and nurses. The questions involved different grades of hemolysis (mild, 1.0, moderate, 2.5 and severe, 5.0g/L) and different final K measurements (2.9, 4.0, 5.2 and 8.2mmol/L). Subjects estimated the K concentration in a non-hemolyzed sample for each scenario. Adjustment values (difference between final hemolyzed K concentration and subject's response) were calculated. For the 132 respondees, the mean correct score was 1.7/12. Mean adjustment values were: mild, 0.43mmol/L (K 2.9), 0.55 (4.0), 0.88 (5.2) and 1.53 (8.2); moderate, 0.85 (2.9), 0.92 (4.0), 1.33 (5.2) and 2.50 (8.2); and severe, 0.93 (2.9), 1.48 (4.0), 1.96 (5.2), 2.96 (8.2). Correct adjustments were: mild, 0.28; moderate, 0.70; and severe, 1.40mmol/L. Healthcare staff overestimated the effect of hemolysis on potassium measurement and used an incorrect proportional adjustment approach to the problem. Such poor knowledge and faulty thinking could lead to diagnostic delays or misdiagnoses. There is potential for such faulty thinking in all areas of laboratory medicine, and laboratories should review their educational responsibilities and reporting practices in light of this.