Every doctor, nurse and medical student needs to know normal blood test values by heart. Whether you are clerking a patient at 2 am, interpreting results on a ward round, or sitting an OSCE, knowing what is normal — and what is not — is a fundamental clinical skill. This guide covers every essential blood test with both SI units (used in UK, Europe, India, Australia) and conventional units (used in USA, Canada) so it is useful wherever you practice.
- Full Blood Count (FBC / CBC)
- Urea and Electrolytes (U&E) / Basic Metabolic Panel (BMP)
- Liver Function Tests (LFTs)
- Thyroid Function Tests (TFTs)
- Bone Profile
- Coagulation Screen
- Blood Glucose and HbA1c
- Inflammatory Markers
- Cardiac Biomarkers
- Arterial Blood Gas (ABG) — Normal Values
- ABG Interpretation — Quick Reference
- Frequently Asked Questions
- Conclusion
Full blood count, renal function, liver function, thyroid, bone profile, coagulation, blood glucose, inflammatory markers, cardiac enzymes and arterial blood gas — with SI and conventional units, causes of abnormality and clinical pearls.
Full Blood Count (FBC / CBC)
The full blood count — called CBC (complete blood count) in the USA — is the most commonly requested blood test in medicine. It gives a snapshot of the three major cell lines produced by the bone marrow.
| Parameter | SI Units | Conventional (US) | Low / High |
|---|---|---|---|
| Haemoglobin (men) | 130–175 g/L | 13.0–17.5 g/dL | Low: Anaemia / High: Polycythaemia |
| Haemoglobin (women) | 120–155 g/L | 12.0–15.5 g/dL | Low: Anaemia / High: Polycythaemia |
| White Cell Count | 4.0–11.0 × 10⁹/L | 4,000–11,000 /µL | Low: Leucopenia / High: Leucocytosis |
| Neutrophils | 2.0–7.5 × 10⁹/L | 2,000–7,500 /µL | Low: Neutropenia / High: Bacterial infection |
| Lymphocytes | 1.0–4.0 × 10⁹/L | 1,000–4,000 /µL | Low: Viral/steroids / High: CLL, viral |
| Platelets | 150–400 × 10⁹/L | 150,000–400,000 /µL | Low: Thrombocytopenia / High: Thrombocytosis |
| MCV | 80–100 fL | 80–100 fL (same) | Low: Microcytic / High: Macrocytic anaemia |
MCV is your most important clue when investigating anaemia:
- Microcytic (MCV <80 fL) — Iron deficiency anaemia, thalassaemia, anaemia of chronic disease
- Normocytic (MCV 80–100 fL) — Acute blood loss, haemolysis, renal failure, anaemia of chronic disease
- Macrocytic (MCV >100 fL) — B12 or folate deficiency, alcohol, hypothyroidism, liver disease, methotrexate
Neutrophils <0.5 × 10⁹/L (<500/µL) = severe neutropenia — urgent review. Platelets <20 × 10⁹/L (<20,000/µL) = high bleeding risk. Hb <70 g/L (<7 g/dL) — consider transfusion in symptomatic patients.
Urea and Electrolytes (U&E) / Basic Metabolic Panel (BMP)
Called U&E in the UK and BMP or CMP in the USA. These assess renal function and electrolyte balance — among the most frequently deranged results on the wards.
| Parameter | SI Units | Conventional (US) | Key Causes of Abnormality |
|---|---|---|---|
| Sodium | 135–145 mmol/L | 135–145 mEq/L (same) | Low: SIADH, heart failure. High: dehydration, DI |
| Potassium | 3.5–5.0 mmol/L | 3.5–5.0 mEq/L (same) | Low: diuretics, vomiting. High: AKI, ACE inhibitors |
| Urea (BUN) | 2.5–7.8 mmol/L | 7–20 mg/dL | High: dehydration, GI bleed, renal failure |
| Creatinine | 60–120 µmol/L | 0.7–1.3 mg/dL | High: AKI, CKD. Low: low muscle mass, pregnancy |
| eGFR | >90 mL/min/1.73m² | >90 mL/min/1.73m² (same) | CKD stages 1–5 based on level and duration |
| Bicarbonate | 22–29 mmol/L | 22–29 mEq/L (same) | Low: metabolic acidosis. High: metabolic alkalosis |
Hyperkalaemia >6.5 mmol/L (6.5 mEq/L) or any hyperkalaemia with ECG changes is a medical emergency. Give IV calcium gluconate immediately to stabilise the myocardium, then treat the underlying cause.
Liver Function Tests (LFTs)
LFTs assess liver cell damage, biliary obstruction and synthetic function. Frequently ordered in jaundice, alcohol-related illness, drug monitoring and routine clerking.
| Parameter | SI Units | Conventional (US) | Clinical Significance |
|---|---|---|---|
| Bilirubin (total) | <21 µmol/L | <1.2 mg/dL | Jaundice visible at >35 µmol/L (2.0 mg/dL) |
| ALT | 7–56 U/L | 7–56 U/L (same) | Most specific marker of hepatocellular damage |
| AST | 10–40 U/L | 10–40 U/L (same) | Hepatocellular damage; also raised in MI |
| ALP | 30–130 U/L | 30–130 U/L (same) | Cholestasis, bone disease, pregnancy |
| GGT | <50 U/L | <50 U/L (same) | Alcohol use, cholestasis, enzyme induction |
| Albumin | 35–50 g/L | 3.5–5.0 g/dL | Synthetic function marker; low in cirrhosis, malnutrition |
LFT pattern recognition:
- Hepatocellular pattern — predominantly raised ALT/AST → hepatitis, paracetamol overdose, NAFLD
- Cholestatic pattern — predominantly raised ALP/GGT → gallstones, primary biliary cholangitis, drugs
- Mixed pattern — both raised → alcohol-related liver disease, infiltrative disease
Thyroid Function Tests (TFTs)
Thyroid disorders are extremely common and frequently missed. Check TFTs in any patient with unexplained fatigue, weight change, palpitations or mood disturbance.
| Parameter | SI Units | Conventional (US) | Interpretation |
|---|---|---|---|
| TSH | 0.4–4.0 mU/L | 0.4–4.0 µIU/mL (same) | Best screening test for thyroid disease |
| Free T4 | 12–22 pmol/L | 0.9–1.7 ng/dL | Check if TSH is abnormal |
| Free T3 | 3.1–6.8 pmol/L | 2.0–4.4 pg/mL | Active hormone; useful in T3 toxicosis |
How to interpret TFTs:
- Low TSH + High T4 → Primary hyperthyroidism (Graves’, toxic nodule)
- High TSH + Low T4 → Primary hypothyroidism (Hashimoto’s, post-thyroidectomy)
- Low TSH + Normal T4 → Subclinical hyperthyroidism
- High TSH + Normal T4 → Subclinical hypothyroidism
- Low TSH + Low T4 → Secondary hypothyroidism (pituitary cause)
Bone Profile
| Parameter | SI Units | Conventional (US) | Key Causes |
|---|---|---|---|
| Calcium (adjusted) | 2.2–2.6 mmol/L | 8.8–10.4 mg/dL | High: malignancy, hyperPTH. Low: VitD def, hypoparathyroidism |
| Phosphate | 0.8–1.5 mmol/L | 2.5–4.5 mg/dL | Low: malnutrition, refeeding. High: CKD |
| Magnesium | 0.7–1.0 mmol/L | 1.7–2.4 mg/dL | Low: alcohol, diuretics — causes refractory hypokalaemia |
Always correct calcium for albumin: Adjusted Ca = Measured Ca + 0.02 × (40 − albumin) in SI units. In US units: add 0.8 mg/dL for every 1 g/dL albumin is below 4.0. A low albumin makes total calcium appear falsely low.
Coagulation Screen
| Parameter | Normal Range | Clinical Use |
|---|---|---|
| PT / INR | 11–13 sec / INR 0.8–1.2 | Warfarin monitoring, liver synthetic function |
| APTT / aPTT | 25–35 seconds | Heparin monitoring, haemophilia screen |
| Fibrinogen | 2.0–4.0 g/L (200–400 mg/dL) | Low in DIC, liver disease, massive transfusion |
Blood Glucose and HbA1c
Diabetes affects over 500 million people worldwide. These two tests are the cornerstone of diagnosis and monitoring.
| Test | SI Units (mmol/L) | US Units (mg/dL) | Category |
|---|---|---|---|
| Fasting glucose | <6.0 mmol/L | <108 mg/dL | Normal |
| Fasting glucose | 6.1–6.9 mmol/L | 110–125 mg/dL | Impaired (pre-diabetes) |
| Fasting glucose | ≥7.0 mmol/L | ≥126 mg/dL | Diabetes (on 2 occasions) |
| Random glucose | ≥11.1 mmol/L + symptoms | ≥200 mg/dL + symptoms< | |
| Fasting glucose | <6.0 mmol/L | <108 mg/dL | Normal |
| Fasting glucose | 6.1–6.9 mmol/L | 110–125 mg/dL | Impaired (pre-diabetes) |
| Fasting glucose | ≥7.0 mmol/L | ≥126 mg/dL | Diabetes (on 2 occasions) |
| Random glucose | ≥11.1 mmol/L + symptoms | ≥200 mg/dL + symptoms | Diabetes |
| HbA1c | <42 mmol/mol | <6.0% | Normal |
| HbA1c | 42–47 mmol/mol | 6.0–6.4% | Pre-diabetes |
| HbA1c | ≥48 mmol/mol | ≥6.5% | Diabetes |
Inflammatory Markers
- CRP — normal <5 mg/L (same worldwide). Rises within 6 hours. Bacterial infections typically >100 mg/L. Serial CRP more useful than single measurement.
- ESR — normal <20 mm/hr (men), <30 mm/hr (women). Very high ESR (>100 mm/hr) suggests myeloma, GCA or severe infection.
- Procalcitonin — <0.1 µg/L (ng/mL) normal. More specific for bacterial infection. Used to guide antibiotic stewardship in sepsis.
Cardiac Biomarkers
- High-sensitivity Troponin I/T — <14 ng/L (varies by lab and assay). Rises within 3–6 hours of MI, peaks at 12–24 hours. Also raised in PE, myocarditis, CKD and sepsis.
- BNP — <100 pg/mL (ng/L). NT-proBNP — <300 pg/mL (ng/L). Both raised in heart failure, AF, PE and CKD.
- CK (Creatine Kinase) — 24–195 U/L (same worldwide). Raised in MI, rhabdomyolysis, myositis, statin myopathy.
Arterial Blood Gas (ABG) — Normal Values
| Parameter | SI Units | Conventional (US) |
|---|---|---|
| pH | 7.35–7.45 | 7.35–7.45 (same) |
| PaO₂ | 11–13 kPa | 75–100 mmHg |
| PaCO₂ | 4.7–6.0 kPa | 35–45 mmHg |
| HCO₃⁻ | 22–26 mmol/L | 22–26 mEq/L (same) |
| Base excess | −2 to +2 mmol/L | −2 to +2 mEq/L (same) |
| Lactate | <2.0 mmol/L | <18 mg/dL |
ABG Interpretation — Quick Reference
| Condition | pH | PaCO₂ | HCO₃⁻ | Common Causes |
|---|---|---|---|---|
| Metabolic Acidosis | ↓ <7.35 | Normal / ↓ (compensation) | ↓ <22 | DKA, AKI, lactic acidosis, diarrhoea |
| Metabolic Alkalosis | ↑ >7.45 | Normal / ↑ (compensation) | ↑ >26 | Vomiting, diuretics, hyperaldosteronism |
| Respiratory Acidosis | ↓ <7.35 | ↑ >6.0 kPa / >45 mmHg | Normal / ↑ (compensation) | COPD, OSA, opioids, neuromuscular disease |
| Respiratory Alkalosis | ↑ >7.45 | ↓ <4.7 kPa / <35 mmHg | Normal / ↓ (compensation) | Hyperventilation, anxiety, PE, pregnancy, sepsis |
In metabolic disorders — pH and HCO₃⁻ move in the same direction. In respiratory disorders — pH and CO₂ move in opposite directions. Compensation never fully corrects pH back to normal — if it does, suspect a mixed disorder.
Frequently Asked Questions
Why do blood test reference ranges differ between hospitals?
Each laboratory calibrates its analysers against its local population using different reagents. Always check your local lab’s reference ranges rather than relying solely on textbook values.
What is the most important blood test to check first in an unwell patient?
In an acutely unwell patient: blood glucose (immediate bedside), potassium (cardiac risk), lactate (sepsis/shock), and full blood count. These guide your immediate management.
Can a normal blood test rule out serious illness?
No. Troponin can be negative in the first 3–6 hours of MI. Serial testing and clinical judgment are always required.
Conclusion
Knowing normal blood test reference ranges — in both SI and conventional units — is one of the most practical skills in clinical medicine. It allows you to identify abnormalities quickly, prioritise the sick patient, and communicate results clearly across international teams. Use this guide as a daily reference, but always interpret every number alongside the patient in front of you.
References:
1. NICE. Clinical Knowledge Summaries — Blood Tests. Updated 2024.
2. British Society for Haematology. Guidelines for the diagnosis and management of anaemia. 2023.
3. KDIGO. CKD Clinical Practice Guideline. 2024.
4. American Diabetes Association. Standards of Medical Care in Diabetes. Diabetes Care. 2024.
5. ESC Guidelines for the management of acute coronary syndromes. Eur Heart J. 2023.
This guide is reviewed and updated regularly to reflect current clinical guidelines. Bookmark it for quick reference during ward rounds, exams and clinical practice.