How do I read my own lab report?
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Short answer
Read a lab report in this order: the analyte name, the result with its units, the laboratory's own reference interval, and any flag. Units matter, because the same result can be reported as ng/mL or nmol/L. Reference intervals belong to the laboratory that ran the test, so do not compare your value against a range printed by a different laboratory.
A lab report is a table with four columns that matter: what was measured, the result, the units, and the reference interval used by that laboratory. Everything else is administrative. Reading it well is mostly about knowing which comparisons are fair, and knowing that MedlinePlus describes the term normal as misleading, since "it's common for healthy people to sometimes have results outside the reference range".
Units, and why they change everything
The same substance is reported in different units by different laboratories and different countries. Vitamin D appears as ng/mL in the United States and nmol/L in much of Europe, and 30 ng/mL is 75 nmol/L. Cortisol appears as mcg/dL or nmol/L, where 5 mcg/dL is 140 nmol/L. Lipoprotein(a) is reported as mg/dL or nmol/L and the two do not convert reliably. Before comparing your result to anything you have read, check that the units match.
The reference interval belongs to that laboratory
Reference intervals are derived on a specific analytical platform from a specific reference population, with limits set at the 2.5th and 97.5th percentiles, and at least 120 reference subjects are needed to estimate those limits reliably. A TSH interval from one immunoassay does not transfer to another. This is why the same value can be flagged high on one report and unflagged on the next, and why switching laboratories mid-treatment makes a trend hard to read.
Read the series, not the snapshot
One result is a point; three results are information. Line up the same test over time from the same laboratory and look at direction and slope. A creatinine that has moved from 0.7 to 1.0 mg/dL over two years is worth discussing even though both values are normal. A ferritin that has fallen from 90 to 35 mcg/L says something a single value of 35 does not. Keep your own copies, since under the HIPAA Privacy Rule you have the right to inspect, review and receive a copy of your medical records.
| What you see | What it means |
|---|---|
| H or L flag | Outside this laboratory's interval, not necessarily abnormal for you |
| Reference interval in brackets | Central 95 percent of that laboratory's healthy reference group |
| Different units to a previous report | Convert before comparing; do not assume an error |
| Calculated value, such as LDL or eGFR | Derived from other results by an equation with its own limits |
| Comment about assay interference | Repeat before acting, especially with biotin use |
A practical habit: keep a single file of every lab report, note the collection date and time on each, and bring the last three when you see a new clinician. Ask for one sentence per abnormal result explaining whether it changes anything. If the answer is that it does not, write that down too, so the same result does not restart the worry a year later.
The clinical detail
Two report elements are worth knowing about. Calculated results carry the errors of their inputs: LDL cholesterol by the Friedewald equation is unreliable above triglycerides of roughly 400 mg/dL, and eGFR equations assume stable creatinine production, so they are inaccurate in very high or very low muscle mass and during acute illness.
Assay interference is under-recognised. Biotin at supplement doses can shift immunoassay results in either direction; heterophile antibodies can cause spuriously raised results in sandwich immunoassays; macroprolactin produces an apparently high prolactin in an asymptomatic patient; and the hook effect can cause a falsely low result at extremely high analyte concentrations. When a value does not fit the clinical picture, ask the laboratory to repeat it or to run it on a different platform before the number becomes a diagnosis.
Written and medically reviewed by Zahraa Sater, M.D., M.P.H.
Reviewed August 12, 2026 · Next review due August 12, 2028
Sources
- How to Understand Your Lab Results. MedlinePlus, National Library of Medicine
- Reference intervals: current status, recent developments and future considerations. Biochemia Medica. DOI 10.11613/BM.2016.001
- EP28-A3c: Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory. Clinical and Laboratory Standards Institute
- Your Medical Records: Health Information Privacy. US Department of Health and Human Services
This library is educational and is not individual medical advice. Read our Editorial and Medical Review Policy.