All Lab Tests Explained

What a reference range actually tells you

A reference range is not a boundary between healthy and ill. It is a statistical description: for most tests it is the interval containing the central 95% of results from a reference population presumed healthy. By construction, one in twenty perfectly well people falls outside it. Order a panel of twenty tests on someone with nothing wrong and the odds are good that at least one comes back flagged.

This is why a single result slightly outside the range often means less than patients fear, and why doctors repeat borderline tests rather than acting on them immediately. It also explains why the direction and size of the abnormality matter more than the flag itself. A potassium of 5.2 mmol/L and a potassium of 7.2 mmol/L are both reported as high; only one is an emergency.

Why your range may differ from ours

Ranges on this site are the ones most widely used, but yours may legitimately differ. Laboratories derive their own intervals from their own instruments, reagents and local population. Several tests also vary by sex, age, pregnancy and ethnicity in ways a single number cannot capture. Always read your report against the range printed on that report, not against a range you found online, including ours.

Trends usually matter more than single values

For most chronic conditions, the shape of the line over time carries more information than any one point on it. A creatinine that has drifted from 0.8 to 1.1 mg/dL over two years is worth discussing even though both figures sit inside the normal range, because the change is the signal. Conversely, a stable result that has sat just outside the range for a decade is often that person's normal. Keeping your old reports, or asking your practice for them, is one of the more useful things you can do.

Preparation that changes results

• Fasting genuinely matters for glucose and triglycerides, and much less for total and LDL cholesterol than was once believed.
• Timing matters for testosterone, cortisol and iron, all of which follow a daily rhythm and are conventionally sampled in the morning.
• Recent exertion can raise creatine kinase, and sometimes creatinine, for days after unaccustomed heavy exercise.
• Biotin supplements interfere with many immunoassays, including thyroid and troponin tests, and should be mentioned to whoever takes the sample.