How to Read a Routine Blood Test Report
A blood report with a column of red asterisks is alarming to read and frequently misread. Understanding what each value measures, and why reference ranges work the way they do, turns it into useful information.
How reference ranges work
A reference range is usually set to include the middle 95 percent of a healthy population. By definition, 1 in 20 healthy people falls outside it for any given test. Run a panel of 20 tests on a perfectly healthy person and the odds are that one will be flagged.
Ranges also differ between laboratories because of different equipment and methods. Always compare a result against the range printed on your own report, and ideally use the same laboratory for serial testing.
Complete blood count
Haemoglobin. Oxygen-carrying capacity. Low means anaemia; the cause matters more than the number. High can reflect dehydration, smoking, lung disease, sleep apnoea or a bone marrow disorder.
MCV, mean corpuscular volume, is the size of red cells and is the most useful clue to the cause of anaemia. Low suggests iron deficiency or thalassaemia; high suggests B12 or folate deficiency, alcohol, thyroid disease or liver disease.
RDW measures variation in red cell size and rises early in iron deficiency.
White cell count. Raised in bacterial infection, inflammation, steroid use and stress; low in viral infection, some drugs and marrow problems. The differential matters: neutrophils rise in bacterial infection, lymphocytes in viral, eosinophils in allergy and parasitic infection.
Platelets. Low raises bleeding risk and is important in dengue; high occurs with iron deficiency, inflammation and some marrow disorders.
Kidney function
Creatinine. A muscle waste product cleared by the kidneys. Depends heavily on muscle mass, so a bodybuilder and a frail elderly person with the same value have very different kidney function.
eGFR. The estimated filtration rate, a better single indicator. Below 60 persisting for three months indicates chronic kidney disease.
Urea rises with dehydration, a high protein intake and gastrointestinal bleeding as well as with kidney disease.
Urine albumin-to-creatinine ratio. Often omitted and genuinely important: protein leak predicts both kidney progression and cardiovascular risk independently of eGFR.
Electrolytes. Sodium and potassium abnormalities can be dangerous and are often drug-related.
Liver function
ALT and AST. Enzymes released when liver cells are injured. ALT is more liver-specific. Mild elevation is extremely common and most often reflects fatty liver, alcohol or medication. AST also rises with muscle injury, including after heavy exercise, and an AST-to-ALT ratio above 2 suggests alcohol.
ALP, alkaline phosphatase, rises with bile duct obstruction and also with bone disease, growth in children and pregnancy.
GGT rises with alcohol and with several medications, and helps confirm that a raised ALP is of liver origin.
Bilirubin. High levels cause jaundice. An isolated mildly raised unconjugated bilirubin in an otherwise well person, often higher when fasting or unwell, usually means Gilbert’s syndrome, a harmless inherited variant affecting a significant minority of people.
Albumin and prothrombin time reflect the liver’s synthetic function and are better markers of serious liver disease than the enzymes.
Lipid profile
LDL cholesterol. The main treatment target.
HDL cholesterol. Generally protective.
Triglycerides. Rise with refined carbohydrate, alcohol, untreated diabetes and some drugs. Most affected by recent eating.
Non-HDL cholesterol, total minus HDL, captures all atherogenic particles and is increasingly preferred.
Fasting is no longer required for routine lipid testing in most guidelines, though triglycerides are more interpretable fasting.
Blood sugar
Fasting glucose: normal below 100 mg/dL, prediabetes 100 to 125, diabetes 126 or above on repeated testing.
HbA1c: reflects roughly three months of average glucose. Normal below 5.7 percent, prediabetes 5.7 to 6.4, diabetes 6.5 or above. Unreliable in anaemia, after transfusion, in pregnancy and with haemoglobin variants, which is a common source of confusion in India where thalassaemia trait is frequent.
Thyroid
TSH is the first-line test and moves in the opposite direction to thyroid activity: high TSH means an underactive gland. Free T4 is added when TSH is abnormal. Routine T3 testing is rarely needed.
Inflammatory markers
CRP rises within hours of inflammation or infection and falls quickly. ESR rises and falls slowly and is affected by anaemia, age and pregnancy. Both are non-specific: they indicate that something is inflamed, not what.
Common reasons for misleading results
- Recent heavy exercise raising CK, AST and sometimes creatinine
- Dehydration concentrating most values
- Recent meals affecting glucose and triglycerides
- Haemolysis of the sample in the laboratory falsely raising potassium
- Biotin supplements interfering with thyroid and cardiac assays, sometimes dramatically
- Recent infection affecting white cells, ferritin, CRP and glucose
- Time of day for cortisol and testosterone
- Menstrual cycle phase for some hormones
How to use a report sensibly
- Look at the trend across reports, not a single value
- Check how far outside the range a flagged value is; marginal is usually not meaningful
- Interpret results together, not individually
- Repeat an unexpected abnormal result before acting on it
- Bring all previous reports to your appointment
- Ask what the result changes, which is the only question that really matters
Do not start supplements or medication based on a flagged value alone, and do not order broad panels on yourself as a screening ritual; incidental abnormalities lead to anxiety and unnecessary investigation far more often than they lead to useful findings.
This is general information. Blood results must be interpreted by a doctor alongside your symptoms, history and examination.