Variability of laboratory test results

Variabilities of serum total cholesterol and potassium results provided to 11 medical clinics were assessed using an audit sample-split specimen design. This involved collection of 3 tubes of blood from each of 302 patients, with 1 split specimen divided into 3 audit samples: 1 was sent to the original participating laboratory, another to a commercial referral laboratory, and the third to an academic referee laboratory. Two methods were used to assess variability of test results. Method 1 was based on result pairs corresponding to the split specimen and its corresponding audit sample. Method 2 was based on audit sample results only. The 2 methods provided comparable results for total cholesterol; the estimated coefficient of variation was 1.0% to 3.7%. However, method 1 consistently provided higher estimates of variability for potassium; the estimated SD was 0.096 to 0.168 mmol/L for method 1, while it was 0.035 to 0.090 mmol/L for method 2. Method 1 is more practical, but method 2 can provide a more accurate assessment of analytic variability.