Why are pre-operative blood tests needed?
Pre-operative (pre-op) tests serve three purposes: (1) identifying conditions that increase surgical risk and may need treatment before the operation, (2) establishing baseline values to compare against post-operatively, and (3) ensuring safety during anaesthesia. Not every patient needs every test — what is ordered depends on the type of surgery, patient age, and known medical conditions.
Standard pre-operative blood tests
| Test | What it checks | Why it matters for surgery |
|---|---|---|
| CBC (Full Blood Count) | Haemoglobin, platelets, WBC | Low Hb (anaemia) increases transfusion risk; low platelets increase bleeding risk; high WBC may indicate infection |
| Coagulation: PT/INR and APTT | Clotting ability | Bleeding risk; essential if on anticoagulants or liver disease suspected |
| Electrolytes (Na, K, Cl, HCO3) | Fluid and acid-base balance | Low potassium (from diuretics) can cause arrhythmia under anaesthesia |
| Creatinine / eGFR | Kidney function | Impaired kidneys affect drug metabolism and fluid management |
| Fasting glucose | Blood sugar | Diabetics at higher infection risk; glucose management during surgery |
| LFT (albumin, bilirubin) | Liver function | Liver disease affects drug metabolism and clotting |
| Group and Screen / Crossmatch | Blood type + antibody screen | If transfusion may be needed; crossmatch reserves units |
| ECG (not blood test) | Heart rhythm and ischaemia | Baseline for anaesthesia; detects undiagnosed heart disease |
Who needs which tests?
| Patient group | Extra tests recommended |
|---|---|
| Age >50 or known heart disease | ECG, troponin if recent chest symptoms |
| Diabetics | HbA1c, fasting glucose |
| Patients on warfarin / anticoagulants | INR, discuss bridging strategy with surgeon |
| Patients on ACE inhibitors / diuretics | Potassium (low K+ risk) |
| Known kidney disease | Creatinine, eGFR, electrolytes |
| Known liver disease | LFT, coagulation, platelet count |
| Women of childbearing age (major surgery) | Pregnancy test (beta-hCG) |
| Major surgery (bowel, cardiac, vascular) | Full metabolic panel, group and crossmatch, lung function |
Understanding INR before surgery
INR measures how well blood clots. For most operations, surgeons require INR <1.5. If you are on warfarin, your anticoagulation team will advise whether to stop it before surgery, and whether you need bridging therapy with low-molecular-weight heparin (LMWH) injections. Newer anticoagulants (rivaroxaban, apixaban, dabigatran) are typically stopped 24–48 hours before surgery depending on kidney function.
What if blood tests find a problem before surgery?
- Low haemoglobin: operation may be postponed to treat anaemia (oral/IV iron) and reduce transfusion risk
- Abnormal potassium: corrected before anaesthesia to prevent arrhythmia
- Uncontrolled diabetes: HbA1c above 8.5% significantly increases surgical complications; elective surgery is usually deferred
- Unsuspected kidney disease: anaesthetic drugs and contrast dye dosing adjusted
- Unexpected pregnancy: non-urgent surgery deferred
- Abnormal coagulation: haematology input before proceeding
Questions to ask your doctor
- Which specific tests do I need for my type of operation?
- Should I stop my blood thinners before surgery?
- Is my diabetes well enough controlled for safe surgery?
- Do I need to be optimised for anaemia first?
- Is my blood type and crossmatch done?