Which of the following preanalytical errors most commonly causes false increases in serum enzyme measurements?

• A. The patient was not fasting prior to blood draw.
• B. The blood sample was not maintained on ice upon collection and during transport to the laboratory.
• C. The serum was not separated from red blood cells within 1 hour.
• D. The patient smoked three cigarettes just prior to blood collection.

Answer: (C)

Enzyme measurements in serum are highly sensitive to how the sample is handled before analysis. If serum remains in contact with blood cells, enzymes can leak from the cells into the serum, making the activities appear higher than they truly are. The most common way this happens is delaying the separation of serum from red blood cells. If separation isn’t done within about an hour, cellular leakage and other in vitro changes inflate the measured enzyme levels.

To prevent this, blood should be collected in appropriate tubes, allowed to clot, and then promptly centrifuged to obtain serum that is separated from the cells.

Not fasting before the draw usually doesn’t cause a false rise in serum enzyme activities. Smoking a few cigarettes and maintaining temperature (ice) are not as directly impactful on causing a false increase as delaying serum separation; temperature control matters for some labile analytes, but the primary issue for enzyme elevation is prolonged contact with cells.