Effect of laboratory error on the identification of persons with hypercholesterolemia in an employee health service.

Cholesterol is a major risk factor for coronary heart disease. Of 117 employees seen consecutively in the Tennessee State Employee Health Service voluntary screening program, 86 (74%) had cholesterol levels above the reference range reported by a commercial clinical laboratory. This was three times greater than the calculated expected number of 29 (25%, 17 at moderate risk, 12 at high risk for coronary disease). Age and sex adjustment using Lipid Research Clinic guidelines reduced the number with elevated cholesterol to 55 (47%, 24 at moderate risk, 31 at high risk). Split sample cholesterol assays run independently by the commercial laboratory and a university laboratory showed excellent correlation (r = 0.99, commercial laboratory = 1.0 (university laboratory) + 10.8), but a systematic difference of 12.4 mg/dL (SD = 6.4 mg/dL, paired-t = 9.63, p less than 0.00001) between the two laboratories. Further adjustment for this difference reduced the number with elevated cholesterol to 41 (36%, 26 at moderate risk, 15 at high risk). This experience illustrates how small systematic laboratory errors in cholesterol determination can greatly exaggerate the number of persons reported to have clinically important cholesterol elevations. Clinical laboratories should report age and sex adjusted cholesterol reference ranges and provide clients periodic quality assurance reports that their measurements of cholesterol levels are accurate.