Working the panel: calculation, then consistency check

Take the panel from the opening problem and work it the way the laboratory information system does, one step visible at a time. The three measured inputs are TC 210 mg/dL, HDL-C 42 mg/dL, and TG 220 mg/dL. The first check, before any calculation, is whether TG falls below the validity ceiling for the equation in use. At 220 mg/dL, TG is below the 400 mg/dL Friedewald ceiling, so a calculated LDL-C is appropriate to report.

The Friedewald calculation proceeds in two steps. First, estimate VLDL-C as TG/5: 220/5 = 44 mg/dL. Second, subtract HDL-C and the VLDL-C estimate from total cholesterol: LDL-C = 210 - 42 - 44 = 124 mg/dL. Non-HDL-C is a separate, simpler calculation with no triglyceride term: 210 - 42 = 168 mg/dL.

Before release, the laboratory runs one more check that has nothing to do with clinical interpretation: internal consistency. Non-HDL-C must always be numerically greater than or equal to LDL-C, because non-HDL-C is LDL-C plus the cholesterol carried by VLDL and remnant particles. Here, 168 is greater than 124, so the two calculated results are internally consistent with each other. A panel where non-HDL-C came out lower than LDL-C would signal a calculation error, a transcription error, or a data entry problem upstream, not a biological finding, and would need to be corrected before release rather than reported as-is.

The measured and calculated results are total cholesterol 210 mg/dL, HDL-C 42 mg/dL, triglycerides 220 mg/dL, calculated LDL-C 124 mg/dL, and non-HDL-C 168 mg/dL. The method used for the calculated LDL-C result is identified on the report. Clinical risk use and any laboratory comments depend on the applicable clinical context and local policy.

Verify the calculation and the TG ceiling before release, every time, because a calculated result is only as good as the check that confirmed it belonged on the report.