Types of blood tests: the usual NHS tests and what each checks

A request form ticks groups, not single lines

What comes back as thirty lines is usually a handful of small groups that your GP ticked on the request form, and inside each group the lines are meant to be read against each other. Haemoglobin with MCV and ferritin. ALT with AST and GGT. Creatinine with eGFR and how much you had been drinking.

That is why reading your results from top to bottom rarely works, and why each page in this guide explains a group of tests rather than a single marker.

Why one number on its own says little

A single value outside its range, read in isolation, is the quickest route to the wrong conclusion. The same raised ALT does not mean the same thing after a bout of flu, after a heavy weekend, or after starting a new medicine.

So every page here starts from the group: which lines make it up, which one is read first, and which combinations are worth a question at your next GP appointment.

What routine blood tests show, and what they do not

Routine tests describe how a few large systems are behaving on the day of the sample: blood cells, blood sugar, kidneys, liver, thyroid, cholesterol and iron. They are not a cancer screen, they do not look for most infections, and they leave out vitamin D or B12 unless someone asked for them.

More lines do not buy more protection. A Cochrane systematic review of 17 randomised trials and 251,891 participants found that general health checks offered to adults had no effect on deaths from any cause, from cancer or from heart disease. In one of those trials, people offered checks collected 20% more new diagnoses over six years, with no change in outcomes.

Some lines go wrong for practical reasons rather than medical ones. Potassium is the best-known example: red cells damaged in transit release it into the sample on its way to the laboratory. In one NHS pathology service, 82% of critically high potassium results from GP surgeries in November 2024 turned out to be normal on a repeat sample, against 41% in August: cold weather and long journeys to the laboratory were the culprits.

Four results that are read differently today

Four lines are interpreted differently from a few years ago, which matters whenever you compare new results with older ones.

Where to start

If your results run to a long list, begin with the full blood test guide, which explains how the groups fit together and what a check-up can and cannot tell you. Then go to the group that carries your flagged lines: full blood count, urea and electrolytes, cholesterol, liver, kidneys, thyroid or iron.