An Abnormal Test Result: What Happens Next and When It Is Urgent

Your results come back and one line has an asterisk. Usually a single out-of-range value leads to a repeat test and a look at the context, not to treatment: preparation, a recent illness, a supplement or plain chance can all move a number. But when the lab or a doctor calls a result critical, act at once.

When to see a doctor

Right away: an abnormal result together with chest pain, severe shortness of breath, new confusion or fainting; or signs of diabetic ketoacidosis — thirst and frequent urination with vomiting, abdominal pain, drowsiness or fast, deep breathing (NHS, UK). Call 999 in the UK, 911 in the US, 112 in the EU. Also right away, even if you feel fine: the lab or a doctor has told you a result is critical, or the report marks it as critical. Contact the doctor who ordered the test immediately and follow their instructions. If you cannot reach them quickly, do not wait for symptoms: call 111 in the UK or the emergency number elsewhere, and say the lab reported a critical result.

The same day: marked thirst, frequent urination, weight loss and tiredness without the danger signs above — possible type 1 diabetes. In the UK, an urgent GP appointment or 111 (NHS).

Routinely: a single moderate deviation while you feel well. Discuss it with whoever ordered the test and repeat it on their plan.

What is happening

Many things besides disease move a result.

Preparation. The European Federation of Clinical Chemistry and Laboratory Medicine (EFLM) recommends a morning blood draw after 12 hours of fasting, water allowed, with no alcohol for 24 hours and no smoking or caffeinated drinks that morning. Studies of coffee disagree — one latte raised fasting glucose in one study, a cup an hour before the draw changed little in another — so your lab's instructions come first.
Exercise. Hard training raises creatine kinase, a muscle enzyme; before that test you may be asked to skip intense workouts for a few days (MedlinePlus).
The sample. A haemolysed or old sample can show falsely high potassium, which labs rule out before phoning about potassium (Royal College of Pathologists, UK).
Acute illness. Infection, injury, surgery or a heart attack can raise blood sugar for a while; Russian diabetes guidelines, for example, reassess it weeks later rather than diagnose diabetes on the spot. NICE (UK) advises against testing thyroid function during an acute illness unless the thyroid is the suspected cause.
Supplements. Biotin (vitamin B7, common in hair and nail products) can make some tests read falsely high or low, thyroid hormones among them; the FDA (US) warns that a falsely low troponin can miss a heart attack. Tell whoever orders your tests that you take it.

False positives pile up when a condition is rare. Take round, made-up numbers: 1000 people, 10 of whom have a condition. A test catches 9 of the 10 and wrongly flags 5% of the healthy — about 50 of 990. That is 59 positive results, of which only 9 are real: roughly 1 in 6 or 7. The test is not bad; the condition is simply rare. Counting "out of 1000" makes this easier to see than percentages (Gigerenzer et al., 2007).

"Just in case" has a cost. Scans often find something by chance — on some types of scan in more than a third of people — and most such findings are harmless, but not all (O'Sullivan et al., BMJ 2018). Each can start a cascade of further tests: almost all of 376 US internists surveyed had seen one, and many described psychological, physical or financial harm to patients (Ganguli et al., 2019). A Cochrane review of general health checks in adults without symptoms (17 trials, about 250,000 people) found little or no effect on deaths (Cochrane, 2019).

When a deviation is urgent. Labs keep lists of critical values — results that may signal a life-threatening state — and phone them to the responsible clinician without delay: a federal requirement in the US (CLIA), Royal College of Pathologists guidance in the UK. The lists differ between labs and for children, so do not check your result against someone else's table: if your lab or doctor calls it critical, it is. If you have a test done without a doctor's referral, ask beforehand how the lab reports critical results and whom it will contact.

What to do

Keep the report with its ranges and units; how to read them is on how to read a blood test.
List every medicine and supplement for the doctor, biotin included: see supplements without testing and never mix supplements and medications on autopilot.
Ask three questions: what would a repeat change, when and how should it be done, and what result would call for action?
Repeat under the same conditions — same lab, time of day and preparation — and do not treat a number yourself with supplements, diets or medicines before a doctor has seen the whole picture.
Expect some pull toward the average: an unusually high or low first value often comes back closer to typical — behind extreme outcomes lies a return to the mean.

When this is not the case

Symptoms change the order: go by the tiers above, not by how far the value is from the range.
Pregnancy has its own reference ranges; tell the lab and discuss any abnormal value with your midwife or doctor promptly.
Known conditions. With diabetes, kidney disease or a medicine monitored by blood tests, your doctor's plan for that value comes first. Very low blood sugar in someone with diabetes is its own emergency: what to do during a low blood sugar emergency.
Related pages: what to do when you see a scary statistic, check your thyroid if you are always tired, know your blood pressure.

Sources

Checked: 24 September 2026