Abnormal Blood Test Results: Common Causes Explained

A patient once called my clinic in genuine distress after seeing an online portal flag one of her results as “outside normal range” before we'd even had the chance to speak. She'd already imagined the worst by the time we spoke that afternoon. The actual cause turned out to be that she'd gone for a run that morning before her blood was taken, which had temporarily raised one particular marker. Repeated a few days later under normal conditions, it was entirely unremarkable.

I share that story because it’s genuinely one of the most common conversations I have in practice. An abnormal result understandably triggers worry, but the reality is that "abnormal" covers a huge range of possibilities, most of which are minor, temporary, or easily explained, rather than a sign of serious illness.

This article covers:

• Why an abnormal result doesn't automatically mean something is wrong
• The everyday, non-medical reasons blood results can fall outside range
• The medical conditions that more commonly cause abnormal results
• How doctors actually interpret an abnormal result, and when it genuinely needs following up

Quick Answer: What Causes an Abnormal Blood Test Result?

Abnormal blood test results can be caused by a wide range of factors, many of which have nothing to do with serious illness. Common causes include:

• Everyday factors: not fasting when required, dehydration, recent exercise, alcohol, certain medications or supplements, and even the time of day the sample was taken
• Temporary illness: a recent cold, infection, or minor injury can shift markers such as white blood cells or inflammatory proteins
• Underlying health conditions: including thyroid disorders, anaemia, diabetes, liver or kidney problems, and less commonly, more serious conditions
• Statistical variation: reference ranges are set to cover 95% of healthy people, meaning roughly 1 in 20 results in a perfectly healthy person can fall outside the "normal" range purely by chance

A single abnormal result is rarely enough to diagnose anything on its own. Doctors usually interpret it alongside your symptoms, medical history, and sometimes a repeat test, rather than reacting to one number in isolation.

Why "Abnormal" Doesn’t Always Mean Something Is Wrong

Many patients are surprised to learn how reference ranges are actually built. A “normal range” is typically defined as the range within which 95% of a healthy population's results fall, based on statistical distribution rather than a strict biological cut-off between healthy and unhealthy.

• This means that, purely by chance, around 1 in 20 results in a genuinely healthy person can sit outside the reference range
• The more individual markers included in a blood panel, the higher the statistical likelihood that at least one comes back flagged, even in someone with nothing wrong
• A result just outside the range is generally far less significant than one substantially outside it
• Reference ranges themselves vary slightly between laboratories, and by age, sex, and sometimes ethnicity, which is why the same result can occasionally be flagged by one lab and not another

None of this means abnormal results should be ignored, but it does explain why a single flagged marker on a broad private blood test panel often turns out to be nothing significant once properly reviewed.

Common Non-Medical Causes of Abnormal Results

Before the Test

Not Fasting When Required

Tests such as glucose and cholesterol are particularly sensitive to recent food intake.

Dehydration

Can concentrate the blood and temporarily raise markers such as urea and sodium.

Time of Day

Some hormones, including cortisol and testosterone, naturally fluctuate throughout the day.

Recent Exercise

Can raise markers such as creatine kinase, a muscle enzyme often mistaken for a heart or muscle problem.

Alcohol in the Days Beforehand

Can temporarily raise liver enzymes even without ongoing liver disease.

Medications and Supplements

• Many common medications affect blood results, sometimes predictably and harmlessly, including statins (liver enzymes), diuretics (electrolytes), and metformin (vitamin B12 levels over time)
• Over-the-counter supplements, particularly high-dose biotin, can interfere with certain hormone assays, including thyroid tests, producing misleading results
• It's always worth telling whoever is arranging your test about any medications or supplements you're taking, since this context genuinely changes how a result should be interpreted

Recent Minor Illness

• A cold, minor infection, or even recent vaccination can temporarily raise white blood cell counts or inflammatory markers such as CRP
• These changes are usually short-lived and resolve within a couple of weeks once the underlying illness has settled

Common Medical Causes of Abnormal Blood Results

Infections and Inflammation

Raised white blood cells or inflammatory markers most commonly reflect the body responding to infection, whether that’s something as minor as a cold or something requiring more active treatment. The degree of elevation, alongside symptoms, generally points toward how significant this is.

Thyroid, Liver, and Kidney Function

• An underactive or overactive thyroid is one of the more common causes of abnormal hormone results, often alongside symptoms such as fatigue, weight change, or altered mood
• Raised liver enzymes can reflect anything from fatty liver disease and alcohol use to certain medications or, less commonly, viral hepatitis
• Abnormal kidney function markers can reflect dehydration, certain medications, high blood pressure, or diabetes affecting the kidneys over time

Diabetes and Blood Sugar

Raised blood glucose or HbA1c can indicate prediabetes or diabetes, though isolated mild elevations, particularly after a non-fasting sample, don't necessarily confirm this without repeat testing. This is one of the most common reasons for a follow-up diabetes testing appointment in my experience.

Anaemia and Blood Cell Abnormalities

Low haemoglobin, iron, or ferritin can reflect anaemia from a range of causes, including heavy periods, dietary factors, or gastrointestinal blood loss. Abnormal white cell or platelet counts have a wide range of explanations, most commonly recent infection, though persistent or unexplained changes deserve proper follow-up.

Less Common but Important Causes

A question I'm frequently asked, understandably with some anxiety, is whether an abnormal result could indicate cancer. It's important to be honest and balanced here. Certain blood markers can occasionally be affected by cancer, and doctors do consider this among the possibilities for unexplained or persistently abnormal results, but the reality is that infections, inflammation, medication effects, and benign conditions are responsible for the vast majority of abnormal results by a considerable margin. Blood tests alone also can't rule out or confirm most cancers, since not every cancer produces a detectable marker, which is why unexplained abnormalities are followed up with a clear plan rather than either dismissed or treated as an emergency.

Common Blood Tests and Possible Causes of Abnormal Results

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This table is illustrative rather than exhaustive, and the same abnormal result can have multiple possible explanations depending on the wider clinical picture, which is why context always matters more than the number alone.

How Doctors Interpret an Abnormal Result

• Degree of abnormality. A result marginally outside range is treated very differently to one significantly outside it
• Trend over time. A single result is a snapshot; a repeat test showing the same pattern, or a clear trend, carries more weight
• Your symptoms and history. An abnormal result that fits a clinical picture is interpreted differently to an incidental finding in someone who feels entirely well
• Whether it fits together. Doctors often look at several markers together rather than one in isolation, since patterns across a panel are usually more informative than a single flagged value

This is genuinely one of the more valuable parts of a a private GP appointment or an online GP consultation after an abnormal result, having someone properly interpret the number in context, rather than reacting to a flag on a printed report alone.

Common Myths and Misconceptions

Myth: An abnormal result always means I'm seriously ill.

Not true. Most abnormal results have benign, temporary, or easily treatable explanations, and doctors interpret them alongside your wider health picture rather than in isolation.

Myth: If one marker is abnormal, my doctor will definitely find something wrong.

Not necessarily. Repeat testing after addressing an obvious cause, such as improved hydration or fasting properly, often shows the result has normalised.

Myth: A result within the normal range means everything is fine.

Reference ranges are population-based averages, and results at the very edge of "normal," alongside symptoms, can still be clinically relevant and worth discussing.

Myth: I don't need to mention supplements before a blood test.

Certain supplements, particularly high-dose biotin, can genuinely distort specific test results, so it's always worth mentioning what you're taking.

Myth: Repeating a blood test is pointless if it's already been done once.

A repeat test, especially after correcting an obvious factor like fasting or hydration, is often the most useful next step, not a waste of time.

Benefits and Limitations of Blood Testing

Blood tests are a genuinely valuable, relatively low-risk way to screen for and monitor a wide range of health conditions, often picking up problems before symptoms become obvious, which is one of the reasons routine health checks commonly include a broad panel rather than a single test.

Their main limitation is that results need proper context to be meaningful. A single abnormal number, viewed without your history, medications, or a repeat test, can be misleading in either direction, falsely reassuring or unnecessarily alarming. This is why blood tests work best as one part of a wider clinical assessment, not as a stand-alone verdict on your health.

When to See a GP About an Abnormal Result

• If a result is significantly outside the reference range, rather than marginally so
• If an abnormal result is accompanied by symptoms that concern you
• If a result remains abnormal on repeat testing after addressing obvious factors such as hydration or fasting
• If you're unsure what a result means or whether it needs following up at all
• If an abnormal result was flagged through an online portal before you've had the chance to discuss it with a clinician

If something needs closer investigation, this might mean further blood work, or occasionally a referral to a specialist for a more detailed assessment, such as a hepatologist for persistent liver abnormalities or an endocrinologist for thyroid results that don’t fit the usual pattern. It’s also worth having relevant risk factors reviewed together where appropriate, for example alongside cholesterol testing if cardiovascular risk is part of the picture, or cancer screening where genuinely indicated by your age, symptoms, or family history, rather than for every abnormal result by default.

Key Takeaways

• Abnormal doesn’t automatically mean something serious. Statistically, around 1 in 20 results in a healthy person can fall outside the reference range by chance alone.
• Everyday factors, including fasting status, hydration, exercise, alcohol, medications, and supplements, are common and often overlooked causes of abnormal results.
• Doctors interpret results based on degree of abnormality, trends over time, and your wider clinical picture, not a single number in isolation.
• Common medical causes include infection, thyroid or liver conditions, diabetes, and anaemia, with more serious causes being comparatively rare.
• A repeat test, ideally after correcting any obvious contributing factor, is often the most useful next step for a mildly abnormal result.
• If a result is significantly abnormal, persistent, or accompanied by symptoms, it’s worth a proper clinical assessment rather than searching for answers online.

Conclusion

An abnormal blood test result is rarely the dramatic finding it can feel like in the moment, and in my experience, most turn out to have a straightforward explanation once properly reviewed. That said, “usually nothing serious” isn't the same as “never worth checking,” so a result that's significantly abnormal, persistent, or paired with symptoms genuinely deserves a proper conversation with a GP rather than assumptions in either direction. If you've had a result flagged and aren't sure what it means, that uncertainty itself is a good enough reason to get it explained, and services such as Private Medical Clinic can help arrange that review promptly if you'd rather not wait.

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