Blood Tests Before Surgery: The Complete Pre-Op Guide
Pre-operative blood tests help your surgical team identify hidden risks, plan for potential complications, and ensure your body is ready for surgery. Here's what each test checks and what happens if results are abnormal.
When Are Pre-Op Blood Tests Done?
Pre-operative blood tests are typically ordered 2\u20134 weeks before elective surgery, during your pre-admission clinic (PAC) appointment. This timing allows enough time to detect and correct problems (such as anaemia or uncontrolled diabetes) before the operation. For emergency surgery, blood tests are drawn on admission and results are needed urgently \u2014 the lab prioritises these samples.
Elective Surgery Timeline
Pre-admission appointment 2–4 weeks before surgery. Blood tests, ECG, chest X-ray (if indicated), and anaesthetist review. Results reviewed within days. If abnormalities found, there is time for correction. Repeat blood tests may be needed closer to surgery if treatment was initiated (e.g., iron infusion for anaemia).
Emergency Surgery Timeline
Blood drawn on presentation. FBC, UEC, coagulation, group and crossmatch ordered stat. Results available within 1–2 hours. Blood bank prepares units while tests run. O-negative blood available immediately for life-threatening haemorrhage if crossmatch not yet complete. Surgery proceeds based on clinical urgency, even if not all results are back.
The Standard Pre-Operative Blood Tests
Full Blood Count (FBC)
What it measures: The FBC is the most important pre-operative blood test. It assesses oxygen-carrying capacity (haemoglobin), clotting potential (platelets), and infection risk (white blood cells). Haemoglobin is critical because anaemia significantly increases the risk of post-operative complications including wound infection, cardiac events, blood transfusion, and prolonged hospital stay. Pre-operative anaemia is present in approximately 30–40% of surgical patients and is one of the most modifiable risk factors for poor surgical outcomes.
Normal ranges: Haemoglobin: 130–175 g/L (males), 115–160 g/L (females). Platelets: 150–400 ×10⁹/L. WBC: 4.0–11.0 ×10⁹/L. MCV: 80–100 fL. Pre-op targets: Haemoglobin above 130 g/L (males) or 120 g/L (females) for elective surgery. Platelets above 50 ×10⁹/L for most procedures, above 80 ×10⁹/L for neuraxial anaesthesia.
Why it's needed: Detects anaemia (the single most important modifiable pre-op risk factor), thrombocytopenia (increased bleeding risk), leukocytosis (possible occult infection), and polycythaemia (increased thrombosis risk). MCV helps characterise anaemia type: low MCV suggests iron deficiency, high MCV suggests B12 or folate deficiency.
If abnormal: Haemoglobin below 100 g/L in elective surgery: surgery may be postponed for iron supplementation or iron infusion (typically 2–4 weeks). Severe anaemia (Hb below 70 g/L): blood transfusion may be required before proceeding. Platelets below 50 ×10⁹/L: bleeding risk is significantly elevated; haematology consultation required. WBC above 15 ×10⁹/L: investigate for infection before proceeding.
Urea, Electrolytes & Creatinine (UEC)
What it measures: UEC assesses kidney function and electrolyte balance, both of which are critical for safe anaesthesia. Potassium is particularly important because both low potassium (hypokalaemia) and high potassium (hyperkalaemia) can cause fatal cardiac arrhythmias under anaesthesia. Creatinine and eGFR estimate kidney function, which determines how the body will clear anaesthetic drugs and manage fluid balance during and after surgery. Many surgical patients are older adults with undiagnosed chronic kidney disease.
Normal ranges: Sodium: 135–145 mmol/L. Potassium: 3.5–5.0 mmol/L. Creatinine: 60–110 µmol/L (males), 45–90 µmol/L (females). eGFR: above 90 mL/min/1.73m² (normal), 60–89 (mild impairment), 30–59 (moderate), below 30 (severe). Urea: 3.0–8.0 mmol/L. Bicarbonate: 22–32 mmol/L.
Why it's needed: Identifies electrolyte disturbances that could cause arrhythmias under anaesthesia, detects undiagnosed kidney disease (affects drug clearance and fluid management), establishes baseline for post-operative monitoring (UEC is routinely checked daily after major surgery), and identifies dehydration or over-hydration.
If abnormal: Potassium below 3.0 mmol/L: surgery postponed for correction (IV potassium, slow infusion). Potassium above 5.5 mmol/L: urgent ECG, repeat sample to exclude haemolysis artefact, treat if genuine. eGFR below 30: anaesthetist must adjust drug doses, avoid nephrotoxic agents, plan for possible post-op dialysis. Sodium below 125 mmol/L: surgery postponed for investigation and correction (rapid correction can cause osmotic demyelination).
Coagulation Studies (PT/INR and APTT)
What it measures: Coagulation studies assess the blood’s ability to clot, which is essential before any procedure involving cutting. PT and INR measure the extrinsic pathway (vitamin K-dependent factors) and are particularly relevant for patients on warfarin. APTT measures the intrinsic pathway and detects haemophilia, heparin therapy, and lupus anticoagulant. Fibrinogen is the final common pathway protein converted to fibrin — low fibrinogen indicates severe coagulopathy or consumptive coagulation (DIC).
Normal ranges: PT: 11–14 seconds. INR: 0.9–1.1 (normal), 2.0–3.0 (therapeutic warfarin). APTT: 25–36 seconds. Fibrinogen: 2.0–4.0 g/L. Pre-op targets: INR below 1.5 for most surgery, below 1.2 for neuraxial anaesthesia. APTT below 40 seconds.
Why it's needed: Detects inherited bleeding disorders (haemophilia, von Willebrand disease), identifies liver disease (impaired clotting factor synthesis), monitors anticoagulant therapy (warfarin, heparin), and establishes baseline before procedures with significant bleeding risk.
If abnormal: INR above 1.5 in warfarin patient: warfarin withheld 3–5 days pre-op, INR rechecked. Some patients receive vitamin K or bridging heparin. APTT prolonged with no anticoagulant use: investigate for lupus anticoagulant, haemophilia, or factor deficiency. Surgery typically proceeds only after haematology review. Fibrinogen below 1.5 g/L: transfusion of cryoprecipitate or fibrinogen concentrate may be required.
Liver Function Tests (LFTs)
What it measures: LFTs assess liver health, which directly impacts how anaesthetic drugs are metabolised and how clotting factors are produced. Albumin is a particularly important pre-operative marker: low albumin (hypoalbuminaemia) is one of the strongest predictors of post-operative complications including wound dehiscence, infection, and prolonged hospital stay. The liver also produces most coagulation factors, so severe liver disease causes coagulopathy.
Normal ranges: ALT: below 40 U/L. AST: below 40 U/L. GGT: below 60 U/L (males), below 40 U/L (females). ALP: 30–110 U/L. Bilirubin: below 20 µmol/L. Albumin: 35–50 g/L. Pre-op target: Albumin above 30 g/L (below 30 is an independent risk factor for post-op complications).
Why it's needed: Detects undiagnosed liver disease (chronic hepatitis, fatty liver, cirrhosis), assesses nutritional status (albumin), predicts drug metabolism capacity (severely impaired in cirrhosis), and identifies patients at increased risk of post-operative bleeding (liver produces clotting factors II, V, VII, IX, X).
If abnormal: Albumin below 30 g/L: consider pre-operative nutritional optimisation (high-protein diet, supplementation) for 2–4 weeks if surgery is elective. ALT/AST above 3 times upper limit: investigate cause before elective surgery (hepatitis screen, ultrasound). Bilirubin above 50 µmol/L with elevated enzymes: possible biliary obstruction requiring urgent investigation.
Blood Glucose and HbA1c
What it measures: Hyperglycaemia during and after surgery significantly increases the risk of wound infection, delayed healing, and cardiac events. Approximately 10–15% of surgical patients in Australia have undiagnosed type 2 diabetes or pre-diabetes, which pre-operative blood glucose testing can identify. HbA1c reflects average blood glucose over the preceding 3 months and is more informative than a single fasting glucose, which can be elevated by pre-surgical stress alone.
Normal ranges: Fasting glucose: 3.5–6.0 mmol/L (normal), 6.1–6.9 mmol/L (impaired fasting glucose), 7.0+ mmol/L (diabetes). HbA1c: below 42 mmol/mol / 6.0% (normal), 42–47 mmol/mol (pre-diabetes), 48+ mmol/mol / 6.5%+ (diabetes). Pre-op target for known diabetics: HbA1c below 69 mmol/mol / 8.5% for elective surgery.
Why it's needed: Detects undiagnosed diabetes or pre-diabetes, assesses glycaemic control in known diabetics (poorly controlled diabetes has higher complication rates), establishes baseline for perioperative glucose management, and identifies patients who need insulin sliding scale protocols during and after surgery.
If abnormal: HbA1c above 69 mmol/mol (8.5%): elective surgery may be postponed for 2–3 months to optimise glycaemic control. Fasting glucose above 12 mmol/L on day of surgery: surgery may be postponed until glucose is below 10 mmol/L. New diagnosis of diabetes: requires GP follow-up for management plan before elective procedure.
Blood Group and Crossmatch
What it measures: Blood group and antibody screen ensure that compatible blood is available if transfusion is needed during or after surgery. The antibody screen detects irregular antibodies (beyond ABO and Rh) that could cause a transfusion reaction. A crossmatch physically tests compatibility between the patient’s blood and donor units. Not all surgeries require crossmatch — low-risk procedures may only need a "group and hold" (type blood and keep sample on file for 72 hours in case crossmatch is needed urgently).
Normal ranges: ABO: A, B, AB, or O. Rh: Positive or Negative. Antibody screen: negative (no irregular antibodies detected). A positive antibody screen requires identification and selection of antigen-negative donor blood, which may delay transfusion by hours.
Why it's needed: Ensures compatible blood is available in case of unexpected haemorrhage, identifies patients with irregular antibodies (up to 3% of population) who need specially matched blood, and establishes baseline for post-operative haemoglobin monitoring.
If abnormal: Positive antibody screen: Blood bank requires additional time to find compatible units — the surgeon and anaesthetist must be informed. Rare antibodies may require sourcing blood from the Australian Red Cross Blood Service interstate. For high-risk surgery, cross-matched units should be physically available in the operating theatre.
Which Tests for Which Surgery?
Not every surgery requires every test. The NICE (UK) guidelines, widely followed in Australia, recommend a risk-stratified approach based on the surgery complexity and the patient's health status.
| Surgery Type | Typical Blood Tests | ECG | Blood Units |
|---|---|---|---|
| Minor (skin lesion, endoscopy) | Often none (unless on anticoagulants or significant comorbidities) | Only if cardiac history | None |
| Intermediate (hernia, laparoscopic cholecystectomy) | FBC, UEC | If age >65 or cardiac risk | Group and hold |
| Major (hip/knee replacement, colectomy) | FBC, UEC, LFT, coagulation, glucose, group and crossmatch | Routine | 2 units crossmatched |
| Cardiac surgery (CABG, valve) | Full panel + troponin, BNP, echocardiogram | Routine | 4–6 units crossmatched |
| Emergency surgery | FBC, UEC, coagulation, glucose, group and crossmatch, lactate | Routine | 2–4 units (may use O-negative before crossmatch available) |
Anticoagulant & Blood Thinner Management Before Surgery
If you take blood-thinning medications, your pre-operative blood tests include coagulation studies (INR for warfarin, APTT for heparin) and your surgical team will provide specific instructions on when to stop and restart medications.
| Medication | Action Before Surgery | Bridging Required? | Restart After Surgery |
|---|---|---|---|
| Warfarin | Stop 5 days before surgery. Check INR day before. | Bridging heparin for high-risk patients (mechanical valve, recent VTE) | Restart evening of surgery or day 1 post-op (low bleeding risk), day 2–3 (high bleeding risk) |
| Rivaroxaban (Xarelto) | Stop 2 days before (3 days if eGFR <30) | Usually NOT needed (short half-life) | Day 2–3 post-op when haemostasis achieved |
| Apixaban (Eliquis) | Stop 2 days before (3 days if eGFR <30) | Usually NOT needed | Day 2–3 post-op |
| Dabigatran (Pradaxa) | Stop 2–4 days before (depends on eGFR) | Usually NOT needed. Idarucizumab (Praxbind) available for emergency reversal | Day 2–3 post-op |
| Aspirin (low dose) | Usually continue (discuss with surgeon) | N/A | N/A (usually not stopped) |
| Clopidogrel (Plavix) | Stop 7 days before | Not usually (discuss with cardiologist if recent stent) | Day 1–2 post-op when safe |
Preparing for Your Pre-Operative Blood Tests
Fast for 10–12 hours before your appointment (water is fine) if glucose or lipids are included
Bring a complete medication list including supplements and over-the-counter drugs
Inform the pathologist if you are on warfarin, DOACs, aspirin, or clopidogrel
Mention any history of bleeding problems, easy bruising, or family bleeding disorders
Tell your GP about any recent infections, dental work, or illnesses
Ask about stopping fish oil, vitamin E, and herbal supplements (some affect clotting)
The blood draw itself takes approximately 5–10 minutes with 3–6 tubes collected
Results are typically available within 1–3 business days (urgent samples within hours)
Related Reading
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Reference ranges sourced from the Royal College of Pathologists of Australasia (RCPA). Anticoagulant management guidance from the Australian and New Zealand College of Anaesthetists (ANZCA). SmarterBlood provides health information and AI-powered blood test analysis. It is not a substitute for professional medical advice, diagnosis, or treatment.