Blood Test Results Explained
Got a result flagged H or L and a few days until your appointment? These guides explain what each test measures, the usual reasons it’s high or low, what happens next, and the numbers that can’t wait.
Quick answer: A result outside the reference range isn’t automatically a problem: ranges cover about 95% of healthy people, so some healthy results land just outside. What matters is how far out it is, your symptoms and the trend. Each guide below lists common causes first, and the numbers that need same-day care.
How to read a result
- The range is on your report. Labs use slightly different methods, so always compare with the range printed next to your result.
- H and L flags mean outside that range, not necessarily abnormal for you. A slightly high or low result is often repeated rather than acted on.
- Trends beat single numbers. A result that’s stable over years means something different from one that’s changed fast.
- Context matters. Recent illness, exercise, pregnancy, medicines and supplements all shift results.
- Units differ by country. Australia, the UK and most of the world use SI units (mmol/L, µmol/L); the US often uses mg/dL. Each guide gives both where it helps.
Results explained
- IronHigh FerritinUsually fatty liver, alcohol or inflammation, not too much iron. When to check for haemochromatosis.
- Blood countLow Platelets (Thrombocytopenia)Normal range, common causes, bruising and bleeding, and why under 20 needs same-day care.
- InflammationHigh CRP (C-Reactive Protein)Mild, moderate and very high results, hs-CRP for heart risk, and whether stress counts.
- LiverHigh ALT (SGPT)ALT and SGPT are the same test. Fatty liver, alcohol and medicines lead the list. The other liver tests, decoded.
- KidneyLow eGFRKidney stages G1–G5, the one-off dips, and when to see a kidney specialist.
- ThyroidHigh TSHSubclinical vs overt underactive thyroid, why it’s repeated, and when treatment starts.
- Blood countLow Neutrophils (Neutropenia)Mild to severe, viruses and medicines, Duffy-null counts, and the fever rule under 0.5.
- InflammationHigh ESR (Sed Rate)Why normal rises with age, what over 100 means, and the giant cell arteritis warning signs.
- HormonesHigh Calcium (Hypercalcaemia)Corrected calcium, why PTH is the next test, and why over 3.5 mmol/L is an emergency.
- VitaminsLow B12 ResultTotal vs active B12, borderline results, and when not to wait for more tests.
- IronLow FerritinThe clearest sign of low iron stores. Common causes, why the cause matters, and when gut checks are needed.
- Blood countHigh Platelets (Thrombocytosis)Usually a reaction to infection, inflammation or low iron. When it’s the bone marrow, and when to worry.
- ElectrolytesHigh Potassium (Hyperkalaemia)Often a damaged sample. Kidney and medicine causes, and why 6.5 or more is an emergency.
- ElectrolytesLow Sodium (Hyponatraemia)Mild, moderate and profound levels, medicine causes, and the symptoms that mean hospital.
- LiverHigh BilirubinUsually harmless Gilbert’s syndrome. Liver and red-cell causes, and which jaundice can’t wait.
- DiabetesHbA1c ResultNormal, prediabetes and diabetes cut-offs in mmol/mol and %, and what can throw the number off.
- HeartHigh Cholesterol ResultLDL, HDL, non-HDL and triglycerides, why your heart risk score matters more, and FH red flags.
Practise reading results. Play Panelysis, the daily blood-test puzzle: a fictional patient, a real-looking panel, three guesses. More in medical games.
Not sure if it’s urgent? In Australia, call healthdirect on 1800 022 222 for free nurse advice, any time. In an emergency, call 000 or your local emergency number.
Common questions
What do H and L mean on a blood test?
H means your result is above the lab’s reference range and L means below it. Reference ranges are set to include about 95% of healthy people, so roughly 1 in 20 healthy people will have any given result just outside the range.
Why do reference ranges differ between labs?
Labs use different machines and methods, and some set ranges for their local population. That’s why you should always compare your result with the range printed on your own report.
Should I worry about a slightly abnormal result?
Usually not. Slightly high or low results are common and often return to normal on a repeat test. Your doctor will look at how far out it is, your symptoms, your other results and any change over time.
Do I need to fast for blood tests?
Most of the tests on these pages don’t need fasting. Iron studies are best done fasting in the morning, and some glucose and cholesterol tests need fasting. Your request form or lab will tell you.
Can I get my results explained here?
These guides give general information, not advice about your own results. Your GP can interpret them with your history and examination, and decide what, if anything, needs doing.
What is Panelysis?
Panelysis is our free daily blood-test puzzle. Each day you read a fictional patient’s results and pick the diagnosis in three tries. It’s a fun way to practise reading panels.