Lab Values

A laboratory reference range is not a definition of health. It is the interval containing the central 95% of results from a reference population — which means one healthy person in twenty falls outside it by construction.

That is why a single value slightly outside a range is a reason to look again rather than a finding, and why the range printed beside your result comes from the laboratory that ran it. Assays and reference populations differ between labs, so a value compared against a range from a different laboratory is two sources of error stacked.

Diagnostic thresholds are different from reference ranges

Some numbers are not reference intervals at all but diagnostic cut-offs set by a guideline body.

A1C is the clearest case: below 5.7% normal, 5.7 to 6.4% prediabetes, 6.5% and above diabetes. Those are administrative boundaries drawn across a continuous scale, which is why 6.4% and 6.5% are not meaningfully different measurements — they are on either side of a line. A diagnosis normally requires two abnormal results, or one plus a confirming test.

What A1C actually measures, and when it lies

Glucose attaches irreversibly to haemoglobin at a rate proportional to concentration. Red cells live about three months, so A1C reflects average exposure over roughly that window — weighted toward the recent past, with the last 30 days contributing about half.

Because it measures red blood cells rather than glucose, anything altering red cell lifespan alters the result without anything changing about blood sugar. Shortened lifespan reads low: haemolytic anaemia, recent blood loss or transfusion, later pregnancy. Lengthened lifespan reads high: iron deficiency, B12 deficiency, splenectomy. Haemoglobin variants — HbS, HbC, HbE traits — interfere with some assays outright, and the lab needs to know.

Averages hide variability

An A1C of 6.2% is compatible with steady readings around 130 mg/dL and with swings between 70 and 200. The average is identical and the clinical picture is not.

That gap is precisely why continuous glucose monitoring reports time in range alongside average, and it is the main limitation of any single summary number.

Trends beat single values

For nearly everything in this section, the direction across several results says more than any one result. A value drifting steadily within the range can be more informative than a one-off just outside it.

Sources: American Diabetes Association Standards of Care in Diabetes; Nathan et al., ADAG study, Diabetes Care; NIDDK laboratory reference material.

Reference information only, not medical advice. Any result belongs with the clinician who ordered it, alongside the rest of your history.