Understanding Your Blood Test Results: What Those Numbers Actually Mean

That envelope arrives from the surgery, or more likely these days, a notification pings on the NHS app. Your blood test results are ready. You open them to find a bewildering array of abbreviations, numbers, and the occasionally alarming red flag marking something as 'abnormal'. Before you spiral into worry or, conversely, dismiss something important, let's walk through what these tests actually tell us about your health.

As a GP, I spend considerable time each day explaining blood results to patients, and I've noticed the same anxieties and misconceptions appearing repeatedly. The truth is, interpreting blood tests requires context that a simple normal range cannot provide. Your age, symptoms, medications, and previous results all matter enormously. A result that's concerning in a 30-year-old might be entirely expected in someone of 70.

The Complete Blood Count Tells Stories

When we request a full blood count, or FBC as you'll see it abbreviated, we're essentially taking a census of your blood cells. The haemoglobin level, often the first thing people notice, reflects your red blood cells' oxygen-carrying capacity. Men typically run between 130-170 g/L, whilst women sit lower at 120-150 g/L, partly due to menstrual losses and different hormone profiles.

But here's what many don't realise: mild anaemia might cause no symptoms whatsoever, whilst someone with 'normal' haemoglobin might feel exhausted if their level has dropped significantly from their personal baseline. I had a marathon runner last month whose haemoglobin of 125 g/L was technically normal for a woman, but represented a significant drop from her usual 145. She'd been told her fatigue was 'all in her head' because her results were within range.

The white cell count gets interesting when we look at the differential, the breakdown of different immune cell types. Neutrophils rise with bacterial infections, whilst lymphocytes often increase with viral illnesses. Though I should mention, this isn't absolute. I've seen plenty of bacterial infections with normal neutrophils and viral infections that send them soaring. Medicine resists neat categories.

Platelets, those tiny clotting cells, normally number between 150-400 x10⁹/L. Low counts might explain mysterious bruising, whilst elevated levels could indicate inflammation somewhere in the body. Interestingly, platelet counts often rise after surgery or trauma as part of the body's repair response. They also creep up in chronic inflammation, which is why we sometimes find elevated platelets in patients with undiagnosed inflammatory bowel disease or hidden cancers.

Liver Function Tests Rarely Test Function

The poorly named liver function tests don't actually measure how well your liver functions. Instead, they detect enzymes that leak from damaged liver cells. ALT and AST, the transaminases, spike when liver cells die. Alcohol, medications, fatty liver disease, and viral hepatitis all cause these rises, but so does intense exercise. I regularly see panic in patients who've had blood tests the morning after a gym session.

Alkaline phosphatase comes from both liver and bone, which confuses interpretation. In teenagers, it's often sky-high due to growing bones. In adults, elevations might indicate bile duct problems or bone disease. GGT helps us differentiate, as it's more specific to the liver and particularly sensitive to alcohol use. Patients often don't realise that 'normal' drinking patterns can elevate GGT. What society considers acceptable and what the liver considers acceptable are quite different things.

Bilirubin, the breakdown product of old red blood cells, causes jaundice when elevated. But Gilbert's syndrome, a harmless genetic variant affecting 5% of the population, causes intermittent bilirubin rises, particularly during illness or fasting. I diagnose Gilbert's syndrome monthly, usually in anxious patients convinced they have liver disease.

The actual functional capacity of your liver is better reflected by albumin levels and clotting tests. The liver manufactures albumin and clotting factors, so when these drop, we know the liver's synthetic function is compromised. This typically occurs only in advanced liver disease, long after the enzymes have been abnormal.

Kidney Function Speaks in Whispers

Kidney disease is insidious, often causing no symptoms until function drops below 30% of normal. The estimated glomerular filtration rate, or eGFR, attempts to quantify remaining kidney function. Above 90 is normal, but here's the catch: a muscular 25-year-old and a frail 85-year-old with identical creatinine levels will have vastly different eGFRs because the calculation adjusts for age, sex, and ethnicity.

Creatinine itself simply reflects muscle breakdown. Bodybuilders often have 'abnormal' creatinine levels despite perfect kidneys. Conversely, a thin elderly person might have seemingly normal creatinine whilst harbouring significant kidney disease. This is why we trend results over time rather than panicking about single readings.

Urea rises with dehydration, high protein intake, or gastrointestinal bleeding, not just kidney disease. The ratio of urea to creatinine helps us determine the cause. When both rise proportionally, think kidney disease. When urea rises disproportionately, consider dehydration or bleeding.

Electrolytes tell us about cellular health and hydration. Sodium rarely strays far from normal; when it does, it's usually serious. Low sodium might indicate excessive water intake, heart failure, or hormonal problems. High sodium suggests dehydration, though diabetes insipidus, a rare condition causing excessive urination, can also be responsible.

Potassium demands respect. Too high, and the heart might stop. Too low, and it might develop dangerous rhythms. Yet common medications like ACE inhibitors raise potassium, whilst diuretics lower it. We're constantly walking a tightrope, balancing cardiovascular protection against electrolyte disturbance.

Cholesterol Numbers Need Nuance

The traditional cholesterol panel divides fats into 'good' HDL and 'bad' LDL cholesterol, but this oversimplifies a complex picture. Total cholesterol below 5 mmol/L is considered ideal, but I've seen heart attacks in people with 'perfect' cholesterol and healthy 90-year-olds with levels over 7.

The ratio of total cholesterol to HDL provides better risk assessment than individual numbers. A ratio below 4 is ideal. Someone with total cholesterol of 6 but HDL of 2 (ratio of 3) has lower cardiovascular risk than someone with total cholesterol of 4 but HDL of 0.8 (ratio of 5).

Triglycerides reflect recent diet more than anything else. That Chinese takeaway three days before your blood test? It's showing up in your triglycerides. For accurate results, we need a proper fast of 12-14 hours, though newer non-HDL cholesterol measurements don't require fasting, making life easier for everyone.

Familial hypercholesterolaemia affects 1 in 250 people, causing extremely high cholesterol from birth. If your LDL exceeds 5 mmol/L despite a healthy lifestyle, especially with a family history of early heart disease, genetic testing might be warranted. I diagnosed a 32-year-old last year whose cholesterol of 9 led to cascade testing that identified affected family members before they developed problems.

Diabetes Markers Beyond Sugar

Fasting glucose provides a snapshot, but HbA1c tells the three-month story. This test measures glucose stuck to haemoglobin, reflecting average blood sugar over your red blood cells' lifespan. Below 42 mmol/mol is normal, 42-47 indicates prediabetes, and 48 or above diagnoses diabetes.

But certain conditions skew HbA1c results. Anaemia, recent blood loss, and pregnancy all affect red cell turnover, potentially giving falsely reassuring results. Conversely, iron deficiency can falsely elevate HbA1c. This is why we sometimes need multiple different tests to confirm diabetes.

The oral glucose tolerance test remains valuable, particularly in pregnancy and borderline cases. Some people have normal fasting glucose but can't handle a sugar load, revealing diabetes that other tests miss. I recently diagnosed gestational diabetes in a woman whose fasting glucose and HbA1c were perfect, potentially preventing serious complications.

Thyroid Tests and Energy Mysteries

Thyroid dysfunction might be the most overdiagnosed and underdiagnosed condition simultaneously. TSH (thyroid stimulating hormone) is the screening test, but it tells us what the pituitary thinks about thyroid hormone levels, not what's actually happening in your tissues.

The reference range for TSH is contentious. Most labs use 0.5-5.0 mU/L, but many specialists argue the upper limit should be 2.5 or 3.0. I've seen patients transform when their TSH dropped from 4 to 2, despite being 'normal' throughout. Conversely, I've seen others feel no different despite dramatic TSH changes.

Free T4 and sometimes free T3 provide the full picture. Some people can't convert T4 to the active T3 efficiently, leaving them hypothyroid despite normal TSH and T4. This is particularly common in chronic illness, severe stress, or nutritional deficiencies.

Thyroid antibodies complicate interpretation further. Positive antibodies indicate autoimmune thyroid disease, which causes fluctuating hormone levels. A patient might be hyperthyroid one month and hypothyroid the next. These patients need careful monitoring and flexible treatment approaches.

Inflammatory Markers and Hidden Fire

CRP and ESR, our standard inflammatory markers, are frustratingly non-specific. They rise with infection, autoimmune disease, cancer, or even obesity. A normal CRP doesn't exclude inflammation, whilst an elevated level doesn't identify its source.

CRP responds quickly, rising within hours of inflammation and falling rapidly with resolution. ESR changes more slowly, taking weeks to normalise after acute illness. This is why we often check both, using their different timescales to understand the inflammatory timeline.

High-sensitivity CRP, a more precise measurement, predicts cardiovascular risk. Levels below 1 mg/L indicate low risk, whilst above 3 mg/L suggests increased danger. But here's the problem: a simple cold can send hs-CRP soaring, making interpretation challenging without clinical context.

Vitamin Deficiencies in the Worried Well

Vitamin D deficiency has reached epidemic proportions in the UK, with our limited sunshine and indoor lifestyles. Levels below 30 nmol/L are deficient, 30-50 insufficient, and above 50 adequate. But optimal levels for immune function and mood might be higher, around 75-100 nmol/L.

Treatment requires proper doses. The 400 IU supplements from supermarkets won't correct deficiency. We typically prescribe 20,000-50,000 IU weekly for several months, then maintain with 1000-2000 IU daily. Toxicity is rare but possible with excessive supplementation, so monitoring matters.

B12 deficiency masquerades as dozens of conditions. Fatigue, pins and needles, memory problems, depression, and even psychosis can result from low B12. The standard serum B12 test is surprisingly unreliable. Levels between 200-500 ng/L occupy a grey zone where deficiency is possible despite 'normal' results. Active B12 or methylmalonic acid testing provides clarity in borderline cases.

Ferritin reflects iron stores, but inflammation falsely elevates it. During infection or inflammation, ferritin rises whilst iron availability actually decreases. This is why we check iron studies comprehensively, including transferrin saturation, when investigating anaemia.

Understanding Abnormal Results

Most abnormal results aren't actually abnormal for you. Reference ranges capture 95% of the healthy population, meaning 5% of healthy people fall outside 'normal'. With 20 tests on a standard panel, you'd expect one abnormal result by pure chance.

Your personal normal might sit at the edges of the population range. Elite athletes often have abnormal results that reflect adaptation, not disease. Conversely, a result within normal limits might represent a significant change from your baseline, indicating early disease.

Trends matter more than absolute values. A creatinine rising from 70 to 110 μmol/L concerns me more than a stable 115. This is why we compare current results with previous ones whenever possible. The NHS app helpfully shows trends, though the graphical displays sometimes alarm patients unnecessarily.

Medications affect numerous tests. Statins elevate liver enzymes and occasionally cause muscle damage reflected in creatine kinase levels. Metformin can lower B12. Proton pump inhibitors affect magnesium and B12 absorption. We factor in your medication list when interpreting results.

When to Worry and When to Wait

Red flags requiring urgent attention include severely low haemoglobin (below 70 g/L), very high potassium (above 6.0 mmol/L), extreme sodium disturbances, or markedly elevated inflammatory markers with symptoms. These warrant same-day medical review.

Mild abnormalities in asymptomatic patients usually merit repeating tests in a few weeks. Many variations resolve spontaneously. Dehydration, recent illness, dietary changes, or laboratory error can all cause temporary abnormalities.

Context is everything. A white cell count of 11 (slightly elevated) in someone with no symptoms means nothing. The same count with fever, confusion, and rapid pulse suggests serious infection. This is why we correlate laboratory findings with clinical assessment.

Taking Control of Your Results

Request copies of all blood tests, creating your personal health archive. Note the date, why tests were requested, and how you felt. This historical record proves invaluable when seeing new doctors or tracking chronic conditions.

Question unexpected results. Laboratory errors occur. Sample contamination, mislabelling, or technical failures can generate false results. If something seems wildly inconsistent with your health status, it probably is. We routinely repeat surprising results before making treatment decisions.

Understand that normal ranges are guidelines, not absolute truth. A good clinician treats patients, not numbers. Some people need their TSH below 1 to feel well, whilst others are fine at 4. Some thrive with vitamin D at 50 nmol/L, others need 100. Medicine is as much art as science.

Frequently Asked Questions

Why do different laboratories have different normal ranges?

Laboratories establish reference ranges using their local population and specific equipment. A London lab's ranges might differ from Manchester's due to population differences and analytical methods. This is why we always interpret results using the ranges from the testing laboratory.

How often should I have routine blood tests?

Healthy adults under 40 rarely need routine bloods without symptoms. From 40-50, consider five-yearly checks. Over 50, annual or biannual testing becomes reasonable, particularly with risk factors like diabetes, hypertension, or family history. Chronic conditions require individualised monitoring schedules.

Can I eat before blood tests?

Fasting requirements depend on the test. Lipid profiles traditionally required 12-hour fasts, though non-HDL cholesterol doesn't. Fasting glucose obviously needs an overnight fast. Vitamins, full blood counts, and most other tests don't require fasting. Always clarify when booking.

Why do my results fluctuate between tests?

Biological variation is normal. Cholesterol can vary by 10% day-to-day. White cell counts change hourly. Hormones follow circadian rhythms. Hydration, recent meals, exercise, stress, and sleep all influence results. This is why we look for patterns, not single readings.

Should I stop medications before blood tests?

Never stop prescribed medications without medical advice. Some tests require medication timing adjustment. Thyroid tests should be done before morning thyroxine doses. Some cholesterol checks might be done before starting statins to establish baseline. Your doctor will advise if medication changes are needed.

Can stress affect blood test results?

Absolutely. Acute stress elevates white cells, glucose, and inflammatory markers. Chronic stress affects cortisol, thyroid function, and sex hormones. Even the stress of venepuncture can elevate certain markers. We consider your psychological state when interpreting results.

What if my GP says results are normal but I still feel unwell?

Normal tests don't exclude disease. Many conditions, including autoimmune diseases, take years to produce abnormal blood tests. Functional disorders cause real symptoms without laboratory changes. Consider seeking second opinions, especially if symptoms persist or worsen.

Are private blood tests worth it?

Private testing offers convenience and comprehensive panels but requires careful interpretation. Some private companies use optimal ranges rather than diagnostic criteria, potentially causing unnecessary worry. Always discuss private results with a qualified clinician who can contextualise findings.

How accurate are home testing kits?

Finger-prick tests are generally less accurate than venous samples. Blood spot tests work well for some markers like vitamin D but poorly for others. Home testing suits monitoring trends rather than diagnosis. Abnormal home results should be confirmed with proper venepuncture.

Why won't my GP test everything I request?

NHS testing follows evidence-based guidelines. Random testing without clinical indication causes more harm than good through false positives, overdiagnosis, and anxiety. Private testing is available for those wanting comprehensive screening, though interpretation challenges remain.

Blood tests provide valuable insights but require intelligent interpretation. They're tools that complement, never replace, clinical assessment. Understanding your results empowers informed health decisions, but remember that wellbeing encompasses far more than laboratory values.

When you next receive results, take time to understand them properly. Ask questions. Request trends. But don't let numbers override how you actually feel. Medicine serves people, not paperwork.

Concerned about blood test results or need comprehensive health screening? The Online GP offers detailed consultations with time to properly explain your results and their implications. Book your appointment via WhatsApp on +44 7399323620 or visit our Marylebone clinic. Same-day availability including weekends.