How to read a lab report

Since April 2021, US labs have been required to release results to patients as soon as they are ready, without waiting for a clinician to review them first. That rule, part of the Cures Act information-blocking provisions, changed who reads a lab report first. For a large share of results, it is now the patient, often days before any conversation.

This page explains the parts of the report. It does not interpret results, because interpreting a result requires knowing the person it belongs to, and that is the conversation this page is meant to help you have.

What is on every report

Formats differ between laboratories, but the content is standardised because the same regulations govern all of them. Every report carries:

• Specimen identifiers, tying the sample to you and to the requisition.
• Collection date and time, when blood was drawn. Distinct from the received and reported times, and the one that matters most for interpretation.
• The analyte name, sometimes with a method or specimen-type qualifier.
• The result and its units.
• The reference interval for that analyte at that laboratory.
• A flag where the result sits outside that interval.
• The performing laboratory, which matters when results come from more than one.

The reference range is not a target

This is the part most often misread, and the misreading runs in both directions.

A reference interval is a statistical description of a reference population, not a statement about health. It is conventionally constructed as the central 95 percent of results from a group of apparently healthy people. That construction has a consequence people rarely have pointed out to them: by definition, 5 percent of healthy people fall outside it. Two and a half percent sit above and two and a half percent below, while being entirely well.

So a single flagged value on a panel is a common finding rather than a rare one. Run enough analytes on a healthy person and something will flag, purely from how the interval was built.

The reverse error is treating a value comfortably inside the interval as a goal achieved. The interval describes where most people are. Whether a particular value is right for a particular person is a clinical judgment involving their history, symptoms, medications and trend, none of which the report knows.

Why two labs give different ranges for the same test

Comparing a result against a range printed on a different report is a reliable way to reach a wrong conclusion. Ranges differ legitimately for several reasons.

Different assay methods. Laboratories use different platforms and reagents, and for many analytes those are not interchangeable. Each laboratory establishes or verifies its own interval for the method it runs, which is why the interval belongs to the method rather than to the analyte.

Different reference populations. Intervals derived from different groups differ, and many are partitioned by age and sex.

Different units. Covered below.

For some analytes this is a small effect. For others it is large. Hormone immunoassays are a well-documented case: the same specimen measured by different assays can produce results that differ enough to change how they would be read, which is why comparing across laboratories without accounting for method is unsafe. The practical rule is to track a marker at one laboratory using one method, and treat a change of laboratory as a break in the series rather than a data point in it.

Units, and the number that looks ten times too big

Most US laboratories report in conventional units. Much of the rest of the world, and some US labs, report in SI units. The same quantity has a different number in each.

Glucose is the familiar example: 90 mg/dL and 5.0 mmol/L are close to the same thing. Cholesterol, creatinine and many hormones have the same split. A result that looks alarming compared with a figure you read elsewhere is frequently a unit mismatch rather than a finding.

Always read the units printed beside the number, and check they match the source you are comparing against. They are on the report for exactly this reason.

The flag column

Flags mark results outside the reference interval. Conventions vary, but the common ones are H for high, L for low, and A for abnormal. Some reports use HH and LL, or a separate critical marker, for values far enough out to warrant immediate contact.

Two things worth knowing about flags.

They are mechanical. A flag means the number fell outside an interval. It carries no judgment about whether that matters for you, and a value one unit outside is flagged identically to one far outside.

They are not a diagnosis, and their absence is not reassurance. A result can sit inside the interval and still represent a meaningful change from your own baseline. This is why a trend is more informative than any single report, and why the direction of travel is usually the more useful question.

Collection time changes the answer

The collection timestamp is the field most often skipped, and for several common markers it determines how the result should be read at all.

• Cortisol follows a pronounced daily rhythm, highest shortly after waking and falling through the day. A cortisol result without a collection time is close to uninterpretable, which is why reference intervals for it are usually stated for a specific time of day.
• Testosterone also varies through the day in men, typically higher in the morning, which is why confirmatory testing is conventionally done on a morning sample.
• Glucose, insulin and triglycerides depend on when you last ate. Whether the sample was fasting is part of the result, not context around it.

If you are comparing two results months apart, check that they were drawn at comparable times of day and in a comparable fed state before concluding anything moved.

What a single result cannot tell you

Two kinds of variation sit between the number and the truth, and neither is an error.

Analytical variation is the imprecision of the measurement itself. Run the same specimen twice and the two results will differ slightly.

Biological variation is the fluctuation within you. Many analytes vary meaningfully day to day, and some hour to hour, in a person whose underlying state has not changed.

Together these mean a small difference between two reports may represent no real change. It also means an unexpected result is often worth repeating before it is acted on, which is a normal clinical response rather than doubt about the laboratory.

What to bring to the conversation

The most useful thing you can do with a report before seeing a clinician is arrive with specific questions rather than a general worry. Questions that tend to be productive:

• Is this flagged value clinically meaningful for me, or is it the kind of result that is common and unremarkable?
• Has it changed from my previous result, and was that drawn under comparable conditions?
• Should this be repeated before we act on it?
• Could anything I am taking affect this measurement?
• Is this result on the same assay as my last one?

That last question sounds technical and is not. It is the question that determines whether your two results are comparable at all.

Related reading: half-life, steady state, and the glossary for the terms that appear on requisitions.