Two years ago your cholesterol was "fine". This year the report says it is a little high. Has something changed, or is it just a different day? That question is the whole reason repeating blood tests over time is useful. A single result is a photograph of one morning. A series of results, taken the same way, starts to show a direction, and the direction is usually what your GP wants to see.
One result is a snapshot
Every marker in your blood moves a little from day to day, even when your health stays exactly the same. Laboratory scientists call this biological variation, and they measure it so they can judge whether the difference between two results is large enough to mean something, or small enough to be ordinary fluctuation (Sandberg et al., 2023). Add the small variation that comes from the test itself, and it becomes clear why one number on its own can mislead in either direction.
This is also why the NHS lists, among the reasons for a blood test, finding out how well a condition is being treated or managed. Following a marker is a different job from looking at it once.
What can move a result
Some changes have nothing to do with your long-term health. The common ones are worth knowing, because they are easy to keep the same next time:
- Food. You might be asked not to eat for up to 12 hours before a cholesterol test (NHS). Eating before one sample and fasting before the next makes the two hard to compare.
- Time of day. Some markers follow a daily rhythm. Testosterone, for example, is best measured between 7am and 10am, and blood iron changes during the day.
- Exercise. Hard training in the days before a sample can raise creatine kinase (CK), a muscle enzyme, and can nudge other markers too.
- Illness. A cold, an infection or an injury can push up C-reactive protein (CRP) and affect other results for a while.
- Medicines and supplements. Some medicines change results on purpose; others do it as a side effect. Biotin, sold in many hair, skin and nail supplements, is a well-known example: in a small study of healthy adults taking 10 mg a day for a week, several common hormone tests, including thyroid tests, gave falsely high or falsely low results (Li et al., 2017).
How often to repeat
There is no single interval that suits everyone, and repeating a test too often can create worry without adding anything. When your GP or another health professional suggests following a marker, a common rhythm is every 6 to 12 months, or sooner after a change such as a new treatment. The NHS gives two everyday examples: after starting iron tablets, your GP may repeat blood tests over the next few months to check your iron is getting back to normal, and people taking thyroid medicine have regular blood tests to check the dose is right.
For a general check-up with no particular concern, many people simply repeat the same tests once a year or at the same season each year. Between 40 and 74, the NHS Health Check invites eligible people every 5 years, and the tests you choose yourself can sit alongside it. Our article on how to choose a blood test check-up explains where to start.
Practical tips for a fair comparison
- Book the sample at a similar time of day each time, ideally in the morning.
- If the first test was taken fasting, fast again; if not, do not.
- Avoid hard exercise for a day or two before, especially if CK or other muscle markers are included.
- If you take biotin, ask your GP or pharmacist whether to pause it before the test and for how long.
- Wait until you have recovered from a cold or infection, unless your GP has asked for the test.
- Where possible, repeat the same test, as different methods can report slightly different numbers.
- For women who have periods, note the day of your cycle, as hormone results depend on it.
