How to read a lab report: reference intervals, flags, units, and the reasons a number can look wrong

A laboratory report is a table designed for clinicians, and the parts that look most alarming to everyone else (the flags, the bold text) are the parts that mean least on their own. This page walks through the columns and the ways a perfectly good result can look bad.

The columns

Test name. Sometimes abbreviated, sometimes in the lab's own house style. "ALT (SGPT)" and "Alanine aminotransferase" are the same test; so are "Creat" and "Creatinine". The glossary covers the abbreviations on this site.

Result. The number.

Unit. Where most confusion starts. US laboratories report glucose in mg/dL; much of the world uses mmol/L, and the numbers differ by a factor of 18. Vitamin D is ng/mL in the US and nmol/L elsewhere (factor of 2.5). B12 is pg/mL or pmol/L. Calcitonin is pg/mL or ng/L, which happen to be identical. If you compare a result to a range, compare the units first.

Reference interval (also "reference range" or "normal range"). The interval the laboratory considers usual for that test on that instrument in its reference population. Most are set as the central 95% of healthy people's results, which means 2.5% of healthy people fall below and 2.5% above by construction. Some intervals depend on age or sex and the report will show the one that applies to you. This is the range that applies to your sample; MedlinePlus says on every test page that normal values vary among laboratories, and the ranges on this site are quoted for orientation only.

Flag. H or L (sometimes an asterisk, an arrow or bold text) marks a result outside the reference interval. It is a comparison, not a diagnosis. Some reports also flag "critical" values, which are results far enough outside the interval that the lab telephones the clinician; those are a different category and your clinician will already know.

Previous results. Many reports print the last one or two results for the same test alongside. This column is more useful than the flag: a creatinine of 1.2 that was 1.2 last year and the year before is a different fact from one that was 0.8.

Comments. Sample quality notes ("hemolyzed", "lipemic", "non-fasting"), method changes, and interpretive text. A hemolyzed sample (red cells broke during collection) falsely raises potassium and some enzymes; a lipemic sample (high fat, usually non-fasting) interferes with several tests. If the comment says the sample was compromised, the flagged result may not be real.

Why a healthy person gets flags

If each test has a 95% interval, the chance that one test falls inside it is 0.95. The chance that all 14 tests on a metabolic panel do is 0.95 to the power of 14, which is about 0.49. So roughly half of healthy people will have at least one flag on a metabolic panel by arithmetic alone. Add a CBC and a lipid panel and the odds of a completely flag-free report in a well person are low. Clinicians know this, which is why they read flags in context rather than one at a time.

What moves a result besides disease

Fasting. Glucose and triglycerides rise after eating. A non-fasting glucose of 130 mg/dL means little; a fasting one is a different matter.

Time of day. TSH varies during the day; MedlinePlus notes a morning draw is best. Cortisol and iron also cycle.

Exercise. Hard exertion in the preceding day or two raises AST, creatine kinase and sometimes creatinine.

Hydration. Dehydration concentrates the blood: hematocrit, hemoglobin, albumin, BUN and creatinine all read higher. On GLP-1 therapy during a week of nausea, this is the pattern the labels ask prescribers to look for, and it is also the pattern that can make a CBC look falsely fine.

Body composition. Creatinine comes from muscle. As lean mass falls with weight loss, creatinine falls and calculated eGFR rises, without any change in the kidney. The kidney page explains why that matters.

Supplements. High-dose biotin (common in hair and nail supplements) interferes with several immunoassays, including TSH and some hormone tests. Recent B12 or vitamin D doses raise those levels. Tell the lab and your clinician what you take.

Sample handling. A tube that sat too long reads low for glucose and high for potassium. A difficult draw hemolyzes. The comments column usually says.

Delta checks: the comparison that matters

The most useful thing you can do with a report is put it next to the last one. A value that has moved from the middle of the interval to its edge is more informative than one that has always sat just outside it. Laboratories run "delta checks" of this kind automatically for some tests; the lab tracker lets you do it for yourself, spaced by date, without interpreting anything.

Bringing a report to a visit

• Bring the whole report, not a photograph of the flagged line.
• Note anything unusual about the draw: not fasting, unwell that week, a hard workout, a new supplement.
• Ask three questions about any flag: is it real, has it changed, and does it change anything.
• Ask which laboratory to use next time, so intervals match.

MedlinePlus puts it in one sentence on every test page, and it is the right sentence to end on: talk to your provider about the meaning of your specific test results.