16.5 — Reading Your Own Blood Report

A "normal range" is defined as the values covering the middle 95 percent of a healthy reference population.

Which means that by definition, 5 percent of perfectly healthy people fall outside it — 2.5 percent above and 2.5 percent below.

Run twenty tests on a completely healthy person and, on average, one will come back abnormal. A standard panel contains more than twenty.

This is the single most important thing to know before reading a blood report, and it explains why a doctor looking at one mildly flagged value in a well person is frequently unconcerned — and is usually right.

What matters is the pattern, the size of the deviation, the trend over time, and the clinical picture.

Full blood count

Haemoglobin — the oxygen-carrying capacity. Men 130 to 170 g/L; women 120 to 155.

Low = anaemia, and the next value tells you what kind.

MCV — mean cell volume, the average red cell size. 80 to 100 femtolitres.

This is the most useful single number in investigating anaemia (Chapter 7.1).

Low MCV (microcytic) — iron deficiency, thalassaemia, chronic disease. Normal MCV (normocytic) — acute blood loss, chronic disease, kidney failure, haemolysis. High MCV (macrocytic) — B12 or folate deficiency, alcohol, liver disease, hypothyroidism, some drugs.

And two practical points.

Alcohol raises the MCV before anything else is abnormal, which makes it a quiet early marker.

And in a man or a postmenopausal woman, iron deficiency means investigating the gut (Chapter 7.1) — this is a rule, not a suggestion.

High haemoglobin — dehydration (falsely, from concentration), smoking, chronic lung disease, living at altitude, or polycythaemia vera, a bone marrow disorder.

White cell count — 4 to 11 × 10⁹/L.

The differential is where the information is.

Neutrophils high — bacterial infection, inflammation, steroids, stress, smoking. Neutrophils low — chemotherapy, some drugs, viral infection, marrow failure. Below 0.5 is neutropenia and a fever is an emergency (Chapter 7.1). Lymphocytes high — viral infection, and in an older person a persistently high count raises the question of chronic lymphocytic leukaemia. Eosinophils high — allergy, parasites, drug reactions, some vasculitides.

Platelets — 150 to 400 × 10⁹/L.

Low — bleeding risk below about 50, and spontaneous bleeding below about 20. Causes include immune destruction, marrow problems, liver disease, and drugs. High — often reactive to inflammation, infection or iron deficiency; occasionally a marrow disorder.

And a false low is common: platelets clumping in the sample tube. A surprising isolated low platelet count in a well person is repeated in a different tube before anything is done.

Urea and electrolytes

Sodium — 135 to 145 mmol/L. A measure of water balance, not of salt (Chapter 10.3).

Potassium — 3.5 to 5.0 mmol/L. The most immediately dangerous value on the report in either direction (Chapter 10.3).

And a falsely high potassium is common, from haemolysis in the sample. A surprising high potassium in a well person is repeated before treatment.

Urea — 2.5 to 7.8 mmol/L. Rises with kidney impairment, dehydration, a high protein intake, and gastrointestinal bleeding, because blood in the gut is digested as protein.

A urea rising out of proportion to creatinine suggests dehydration or a gut bleed rather than kidney disease, which is a genuinely useful discrimination.

Creatinine and eGFR — kidney function (Chapter 10.2).

And the crucial caveat: creatinine is produced by muscle, so a bodybuilder has a high creatinine with normal kidneys and a frail elderly person can have a normal creatinine with badly impaired kidneys. eGFR corrects partially for this and not completely.

A creatinine that has doubled is a halving of kidney function, even if both values look unremarkable.

Liver function tests

And the name is partly a misnomer (Chapter 9.4): some measure damage, and some measure function.

Markers of damage:

ALT — relatively liver-specific. AST — also in muscle and heart.

Raised ALT and AST out of proportion = liver cell injury. Viral hepatitis, drugs, alcohol, fatty liver, autoimmune hepatitis.

And the AST:ALT ratio has a specific use. A ratio above 2 suggests alcohol-related liver disease; below 1 is more typical of fatty liver or viral hepatitis.

ALP (alkaline phosphatase) — from liver bile ducts and from bone.

GGT — from bile ducts, and it distinguishes the two. Raised ALP with raised GGT is liver; raised ALP with normal GGT is bone — growth in adolescents, Paget's disease, bone metastases, vitamin D deficiency.

Raised ALP and GGT out of proportion = a bile drainage problem. Gallstones, stricture, tumour, drugs.

And GGT alone is raised by alcohol and by many drugs, which makes it sensitive and not specific.

Markers of function:

Albumin — 35 to 50 g/L. Made by the liver, half-life around 20 days.

Low albumin — chronic liver disease, protein loss in urine or gut, malnutrition, and inflammation (Chapter 16.3). In an acutely unwell patient it usually reflects inflammation rather than nutrition.

Bilirubin — raised in jaundice (Chapter 9.4).

Prothrombin time / INR — clotting factors have half-lives of hours to days, so this is the most sensitive measure of acute liver function.

And Gilbert's syndrome deserves a mention because it causes a great deal of unnecessary alarm. Affecting around 5 percent of people, it causes a mildly raised unconjugated bilirubin, often noticed during fasting or illness, with entirely normal liver function and no consequence whatsoever. It is a benign variant, not a disease.

Bone and mineral

Calcium — must be corrected for albumin (Chapter 10.3).

High — hyperparathyroidism or malignancy account for the great majority. Low — vitamin D deficiency, kidney failure, after thyroid surgery.

Phosphate, magnesium — and magnesium is under-tested, particularly in people on diuretics or proton pump inhibitors, and a low potassium that will not correct is frequently a magnesium problem (Chapter 10.3).

Vitamin D — deficiency is extremely common at high latitudes.

Thyroid

TSH is the screening test, and it is measured first because the axis amplifies (Chapter 12.1).

High TSH, low T4 = primary hypothyroidism. Low TSH, high T4 = hyperthyroidism. High TSH, normal T4 = subclinical hypothyroidism.

And thyroid tests are unreliable in acutely ill patients (Chapter 12.3), which is why they are generally deferred.

Glucose and HbA1c

Fasting glucose — normal below 6.1 mmol/L; 6.1 to 6.9 is impaired; 7.0 or above on two occasions is diabetes.

HbA1c — average glucose over 2 to 3 months (Chapter 12.5). Below 42 mmol/mol normal; 42 to 47 prediabetes; 48 or above diabetes.

And its limitations matter: unreliable in anaemia, haemoglobin variants, kidney failure and pregnancy.

Lipids

Total cholesterol, LDL, HDL, triglycerides.

And the interpretation has shifted (Chapter 18.9). LDL is the causal driver, established by Mendelian randomisation (Chapter 16.1). Non-HDL cholesterol — total minus HDL — is now often preferred, because it captures all the atherogenic particles.

HDL's role has changed. It remains a good risk marker and drugs that raise it have failed to reduce events, so it is no longer treated as a target.

Very high triglycerides — above 10 mmol/L — cause pancreatitis (Chapter 9.5).

And the number alone does not determine treatment. A cardiovascular risk score, combining age, sex, blood pressure, smoking, diabetes and cholesterol, is what guides it.

Inflammatory markers

CRP and ESR (Chapter 16.3).

And they are non-specific by design. A raised CRP tells you there is inflammation somewhere and nothing about where or why. Using it to decide whether someone has an infection, in isolation, is a common error.

Cardiac markers

Troponin — released from damaged heart muscle. Highly sensitive, and raised in many conditions other than heart attack: heart failure, pulmonary embolism, sepsis, kidney failure, arrhythmia, myocarditis.

Which is why it is interpreted as a rising or falling pattern in the right clinical context, not as a single yes-or-no value (Chapter 18.3).

BNP or NT-proBNP — released when the heart chambers are stretched (Chapter 12.7). A normal level effectively excludes heart failure, which makes it most useful as a rule-out test.

The tests that are frequently misused

Ferritin. A marker of iron stores — and also an acute phase protein (Chapter 16.3). So it rises with inflammation, and a normal ferritin does not exclude iron deficiency in someone who is inflamed.

D-dimer. A normal result usefully rules out clot in someone at low or moderate risk. A raised result means very little, because it rises in infection, inflammation, pregnancy, cancer and after surgery. Ordering it in a high-risk patient is a mistake — it cannot rule out and it will not rule in.

Tumour markers. PSA, CA-125, CA19-9 and the rest are useful for monitoring known cancer and are poor screening tests, because they lack both sensitivity and specificity. CA-125 is raised in endometriosis, fibroids, pregnancy and infection.

And vitamin and mineral panels ordered without indication generate abnormal values that lead to supplements without benefit.

Reading a report sensibly

Six rules.

Look at the pattern, not the individual value. A single flagged number in an otherwise normal panel usually means nothing.

Look at the trend. A creatinine that has doubled within the normal range is more significant than a stable value slightly above it.

Ask how far outside the range it is. Marginal is usually noise; markedly abnormal is not.

Ask whether it fits the clinical picture.

Consider the artefacts — haemolysis, clumping, wrong tube, timing relative to food or drugs.

And ask what would change management. A result that changes nothing did not need to be measured, and the number of tests done for reassurance that instead generate anxiety is substantial.

Two practical rights worth knowing. You are entitled to your own results in most health systems, increasingly through patient portals. And "your results are normal" is a reasonable thing to ask to see rather than simply accept — not from mistrust, but because a copy in your own records is genuinely useful the next time something is measured.

What the next page fixes

Blood tells you about chemistry. Chapter 16.6 covers the tests that show structure — X-ray, ultrasound, CT, MRI and the nuclear scans — what each can and cannot see, what the radiation doses actually are, and why the choice of scan is a real decision rather than a formality.