How to Read a Blood Test

How to read a blood test in five steps

Blood test reports are written for clinicians, so they can look intimidating: columns of abbreviations, numbers, units, ranges and flags. Once you know what each column means, most reports follow the same layout, and you can understand far more than you might expect. Here is the short version of how to read a blood test:

1. Find the test name – the abbreviation on the left, such as Hb, ALT, TSH or eGFR.
2. Read your result and its unit – for example, 13.2 g/dL. The unit matters; the same result can look completely different in another unit.
3. Compare it with the reference range printed next to it – the range where most healthy people's results fall *in that laboratory*.
4. Look at the flag – H (high), L (low), or symbols such as * or ! – and remember that a flag is a prompt to look closer, not a diagnosis.
5. Read results together and over time – patterns across tests and trends across reports are much more informative than any single number.

The blood test reader above helps with steps 2–4. Choose the test from the list, enter your result in the unit shown, and – ideally – the low and high limits printed on your report. It tells you whether the result is inside, above or below the range, by how much, and what a low or high value commonly means. For a full overview of typical ranges, see the normal lab values chart.

Anatomy of a lab report

A typical report row looks like this:

What each column means:

• Test: the name of the measurement, often abbreviated. Panels group related tests: a CBC (blood count), a BMP or CMP (metabolic panel), a lipid panel, thyroid tests, an iron panel and so on.
• Result: your value.
• Flag: H or L if the result is outside the reference range; some labs use HH/LL or "critical" for values that need urgent attention.
• Units: what the number is measured in. More on this below.
• Reference range (also called the reference interval or "normal range"): the range expected in healthy people, specific to that lab's method and population.

Other things you may see: the date and time of collection (important for tests that vary through the day, such as cortisol and iron), whether you were fasting, comments about the sample (such as "haemolysed"), and calculated values such as eGFR, LDL cholesterol or the anion gap.

What a "normal range" really means

Most reference ranges are built by testing a large group of healthy people and taking the middle 95% of their results. By definition, then, 1 in 20 healthy people will fall outside the range for any given test – 2.5% above and 2.5% below – without anything being wrong.

This has an important consequence when many tests are done at once. If you have 20 independent tests, the chance that at least one will be flagged even though you are perfectly healthy is 1 − 0.95²⁰, or about 64%. Mildly abnormal results are therefore extremely common, and a single result just outside the range is often of no significance.

(These figures assume the tests are independent, which is only approximately true, but they illustrate the point well.)

Conversely, a result inside the range is not always "normal for you". People have their own set points, and a value that has moved from the bottom to the top of the range over a few years can be more meaningful than a single flagged result.

Some ranges are not 95% ranges

Not every range on a report is built from healthy people. Some are decision limits or targets derived from outcome studies:

For these tests, the "range" is a judgement about risk rather than a description of what healthy people have, and values can differ between guidelines.

Units: the biggest source of confusion

The same blood glucose can be written as 99 mg/dL (US) or 5.5 mmol/L (most other countries). A cholesterol of 200 mg/dL is 5.2 mmol/L. A creatinine of 1.0 mg/dL is 88 µmol/L. If you compare a result with a range from the internet that uses a different unit, the result can look dangerously high or low when it is not.

Always use the unit printed next to your result. If you need to convert, the lab unit converter covers most common tests, and the mg/dL to µmol/L converter handles creatinine, uric acid, bilirubin and others.

Why your lab's range is the one to use

Different laboratories use different analysers, reagents and calibrations, and they serve different populations. As a result, the reference range for the same test can differ noticeably between labs – ALT and vitamin B12 are well-known examples. Some tests also have ranges that depend on:

• Sex – haemoglobin, creatinine, ferritin, testosterone, uric acid.
• Age – TSH, alkaline phosphatase (high in growing children), creatinine, many hormones.
• Pregnancy – haemoglobin, TSH, creatinine, ALP.
• Time of day – cortisol, iron, testosterone.
• Menstrual cycle phase – oestradiol, progesterone, LH, FSH.

This is why the reader above asks for your lab's limits when you have them. When you enter them, they override the general ranges.

Common blood test abbreviations

If a test on your report is not in the reader above, the NBME lab values reference lists over a hundred analytes with their standard ranges in both unit systems.

Reading results together: patterns

A single number rarely tells the full story. Clinicians look for patterns across related tests:

Trends matter more than one result

Every measurement has some variation: analytical variation from the instrument and biological variation from your body's natural day-to-day changes. For some tests this variation is large. Creatinine can vary by around 5–10% between tests in a stable person; triglycerides and some hormones vary much more. A change between two results is only meaningful if it is bigger than this combined variation.

Useful habits:

• Keep your results in one place (most patient portals do this for you) and look at the trend line.
• Compare like with like: same lab, similar time of day, same fasting state.
• Look at the size of the change, not just whether the flag appeared or disappeared.
• Repeat a single unexpected result before drawing conclusions – which is exactly what clinicians usually do.

Common reasons for a misleading result

Questions to ask your clinician

• Which of these results are important, and which are minor variations?
• Do any results need repeating, and when?
• How do these compare with my previous results?
• Could any of my medicines or supplements affect these tests?
• Is there anything I should do differently before my next test (fasting, timing, exercise)?
• What symptoms should make me get in touch sooner?

When to seek help quickly

Most flagged results are not urgent. But contact a clinician promptly – or seek urgent care – if your report is labelled critical, or if an abnormal result comes with symptoms such as chest pain, breathlessness, confusion, fainting, severe weakness, bleeding that will not stop, black stools or very little urine. Laboratories phone the requesting clinician about critical values, but if you see one first, do not wait.