Abnormal blood test result: what does it mean?

An abnormal blood test result, marked with an asterisk, an arrow or in bold, is not a diagnosis. It indicates that the figure falls outside the laboratory's reference range, a range calculated from healthy people which, because of how it's built, leaves out some of them. What matters is how far off it is and whether it fits with the rest of the test and your situation, and that is for a doctor to assess.

What is the reference range on a blood test?

Next to each result, the report shows two figures: the lower limit and the upper limit. This range is obtained by testing a large number of healthy people and usually covers the vast majority. In other words: around 5 in every 100 healthy people have a result outside the range on any given test, half above and half below.

This has two practical consequences:

• The more tests a blood test includes, the more likely it is that one will be flagged even if everything is fine.
• Being within the range doesn't guarantee there is nothing wrong either: ranges don't separate health from illness one hundred per cent.

Why the range differs from one laboratory to another

Each laboratory sets or validates its ranges according to its methods, equipment and units. That's why the same test can have different limits on two reports, and the range can also vary with age, sex or pregnancy.

The practical thing is to read each result against the range printed next to it, not a table found online. And if you want to track a figure over time, it helps to use the same laboratory and keep your previous reports.

Factors that shift a result without any illness

Many figures change because of circumstances on the day the blood is taken:

• Recent food or not fasting for long enough, which mainly affects glucose and triglycerides. How to prepare is explained in the article on fasting before a blood test.
• Intense exercise in the days before.
• Medicines, vitamins and supplements, including those bought without a prescription.
• Alcohol the night before or a very heavy meal the day before.
• Menstrual cycle, for some tests.
• Hydration: being dehydrated concentrates the blood.
• Time of day, because some values change over the course of the day.
• A recent infection or illness, even a mild one.

If any of these things happened before your blood was taken, mention it when you go over the report: it may explain a small deviation.

How an abnormal result is interpreted

Whoever reviews a blood test doesn't look at isolated figures, but at four things at once:

• How far off it is. Being just above the limit is not the same as being double it.
• The overall picture. Some values are related to each other; an isolated abnormality carries different weight from several that point in the same direction.
• The trend. Comparing with previous blood tests shows whether the figure is stable, rising or falling.
• Your context. Why the test was requested, what symptoms you have, what medication you take and your medical history.

That's why the same figure may be of no importance in one person and need following up in another. Looking up an isolated value online leaves out exactly what gives the result its meaning.

When a blood test is repeated

Repeating the test is a common tool, not a warning sign. It's usually considered when:

• The result is inconclusive or there are doubts about whether it's correct.
• The blood wasn't taken under the right conditions, for example without the necessary fasting or after intense exercise.
• The aim is to check whether a slight deviation persists or disappears.
• Progress needs to be followed after a change of treatment.

When to repeat it and what to include is for the doctor to decide. Repeating blood tests on your own, without anyone interpreting them, tends to raise more questions than answers.

Who interprets the report?

The result is interpreted by the doctor who requested the test, who knows the reason for it and your history. If you had the blood test done on your own initiative, take it to your family doctor, who can assess it in context and decide whether anything more is needed. The full blood test page explains what each section of a general blood test measures.

It's best to bring the full report to your appointment, not just the sheet with the flagged figures, along with your previous blood tests and the list of medication you take.

What to ask when going over your blood test

A few well-chosen questions help you leave the appointment knowing what your result means and what to do next:

• Does this figure fit with the rest of the blood test and with what's happening to me?
• Could it be due to something on the day the blood was taken or to the medication I take?
• What was it like on my previous blood tests?
• Does it need repeating, or do I need another test? When?
• Do I need to change anything in the meantime, or is monitoring it enough?

Noting down the answers will be useful the next time you see that same figure flagged.

When not to wait for your appointment

A blood test doesn't replace what you're feeling. If you have chest pain, difficulty breathing, a faint, significant bleeding, confusion or severe discomfort that's getting worse, go to A&E or call 112, whether or not you have results pending. And if the laboratory or your doctor contacts you about a result, follow their instructions.

For everything else, the route is simple: have your blood taken in the right conditions at the clinical laboratory service and go over the report with a doctor who can put it in context.