Why Quest and Labcorp show different reference ranges for the same test
You had blood work at Quest one year and at Labcorp the next, and the “normal” range for the same test is different. Nothing is wrong: each lab sets its own reference range. Here is why, and how to compare your results anyway.
What a reference range is
A reference range (or reference interval) describes where most healthy people tested by that lab fall, usually the middle 95%. That has two consequences:
- By definition, about 1 healthy person in 20 falls outside the range for any given test. A result slightly outside is not necessarily a problem.
- A result inside the range is not necessarily optimal: the range describes a population, not what is best for you.
Why two labs give different ranges
- Different instruments and methods. Labs use different analyzers and chemistry (for example immunoassay or mass spectrometry for hormones), which can give systematically different numbers for the same blood sample.
- Different reference populations. Each lab derives or adopts its range from a group of people, and those groups differ.
- Guideline-based limits. For some tests the limit comes from medical guidelines rather than from a population (for example LDL cholesterol or hemoglobin A1c), and labs may present them differently.
- Age and sex. Many ranges depend on age and sex, and labs split them differently.
Real examples
| Test | Quest Diagnostics | Labcorp |
|---|---|---|
| Glucose (fasting) | 65–99 mg/dL | 70–99 mg/dL |
| Testosterone, total (adult men) | 250–1100 ng/dL | 264–916 ng/dL |
A fasting glucose of 68 mg/dL is “normal” at Quest and “low” at Labcorp. Labcorp's testosterone range comes from a study of healthy, non-obese men aged 19 to 39, which is narrower than older ranges.
How to compare results across labs
- Judge each result against its own lab's range. That is what the H and L flags on your report do.
- Follow trends within the same lab when you can, especially for method-dependent tests: testosterone, estradiol and other hormones, vitamin D, thyroid tests, insulin, ferritin, PSA and Lp(a). Results for the same sample can differ by 10–30% between methods.
- Watch the units. Lp(a) is reported in nmol/L by some labs and mg/dL by others, and the two cannot be reliably converted.
- Look at several results, not one. Every test also varies naturally from day to day.
UnitedMarkers keeps each result with its own lab's range, flags it against that range, and when your labs disagree it can draw each lab's range on the chart over time.
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This guide is educational and does not replace advice from a healthcare professional.