Reading a laboratory report: reference ranges, units and flags

The parts of a typical report

Laboratory reports look different from one laboratory to the next, but most contain the same building blocks. Knowing what each block is for makes a report far less intimidating, even though interpreting the result for your health is the job of your doctor.

• Patient details and a sample identifier, which link the result to you.
• The sample type (blood, serum, plasma, urine) and the date and time it was collected — not only the date it was reported.
• The test name, sometimes with the method used to measure it.
• The result and its unit of measurement.
• The reference interval, often labelled reference range or biological reference interval.
• A flag such as H (high), L (low) or a mark for a critical value.
• Comments from the laboratory, and the name of the person who authorised the report.

What a reference interval really means

A reference interval is usually built from measurements of a group of healthy people and set so that it contains the middle 95 percent of their results. By definition, then, about one healthy person in twenty will have a value just outside the interval for any single test. A result slightly above or below the range is not automatically abnormal, and a result inside the range does not by itself rule out a problem.

Reference intervals also vary. Different laboratories use different instruments and methods, so the interval printed on one laboratory’s report may not apply to a result from another. Many tests have different intervals by age and sex, and some change in pregnancy. That is why the interval printed next to your own result is the one to look at, and why comparing numbers from different laboratories needs care.

Units and conversions

The same substance can be reported in different units, and a number without its unit can be badly misunderstood. Blood glucose, for example, is reported in milligrams per decilitre (mg/dL) in India and some other countries, and in millimoles per litre (mmol/L) in many others. Dividing a glucose value in mg/dL by about 18 gives the value in mmol/L, so 126 mg/dL is roughly 7.0 mmol/L.

Creatinine, a marker used to estimate kidney function, may appear in mg/dL or micromoles per litre (µmol/L); one mg/dL is 88.4 µmol/L. HbA1c, which reflects average blood glucose over roughly the previous two to three months, may be given as a percentage or in mmol/mol. Always read the unit printed on the report rather than assuming it, especially when a result from another country or laboratory is being compared.

High, low and critical flags

Most reports flag results outside the reference interval with H or L, or with bold text or an arrow. These flags are automatic comparisons with the interval and do not know anything else about you — your medicines, your other results or your symptoms. Your doctor weighs a flagged value against that wider picture.

Some laboratories also mark critical values: results far enough from the expected range that they may need prompt attention. Laboratories usually try to contact the requesting doctor directly about these. If you see a result marked as critical and have not heard from your doctor, contact them or seek care promptly rather than waiting for a routine appointment.

Why the trend matters more than one value

A single result is a snapshot. Many values move a little from day to day, and every measurement carries some analytical variation. Repeated results over months show a direction: a kidney marker slowly rising, a haemoglobin steadily falling, a cholesterol level coming down after a change in treatment. Those trends are often more informative than whether one value sits just inside or outside the interval.

Trends are easiest to read when tests are done in the same laboratory, with the same preparation, at a similar time of day. Keeping your reports together in date order makes it possible for a doctor to see the pattern quickly.

Things that can change a result

Several everyday factors influence results, which is why laboratories give preparation instructions.

• Fasting: tests such as fasting glucose and some lipid measurements are interpreted differently if you have eaten.
• Time of day: some hormones, such as cortisol, follow a daily rhythm.
• Recent strenuous exercise, dehydration or an acute illness can shift several values temporarily.
• Medicines and supplements: some affect results directly, and high-dose biotin supplements are known to interfere with certain laboratory immunoassays, including some thyroid and cardiac tests. Tell the laboratory and your doctor what you take.
• Sample problems: a haemolysed sample, where red cells have broken during collection or transport, can falsely raise potassium and some other values. Laboratories often note this in the comments.

Questions worth asking your doctor

It is reasonable to ask why a test was ordered, whether a flagged result matters for you, whether it should be repeated, and what would change if it stays the same or gets worse. Ask whether any preparation is needed before the next test, and keep the answers with the report.

What not to do with a report

Do not start, stop or change the dose of a medicine because of a single result you have read yourself. Do not rely on an internet search to decide whether a value is dangerous. A report is one piece of evidence; its meaning depends on your history, your examination and your other results, which is exactly what a consultation is for.

This guide is general information about how laboratory reports are laid out. It is not medical advice. If you are worried about a result, speak to a qualified clinician.