Patient Guide

Blood Tests Before Surgery

Name: Blood Tests Before Surgery: What and Why | HealthReview
Creator: Suman Konda
Published: 2026-07-05

Pre-operative blood tests assess your fitness for anaesthesia and surgery, identify conditions that need treatment beforehand, and ensure blood is available if needed.¹

Written by Suman Konda, PharmD, Clinical Pharmacist · Editorial policy · Not medical advice

Last updated: 01 August 2026 · How we check our content

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?

References

Sources cited on this page. PubMed links open the original abstract.

Pasternak LR. Preoperative evaluation. Anesthesiol Clin North America. 2004;22(1):1–14. PMID 15137069 · doi:10.1016/S0889-8537(03)00117-5

Why pre-operative testing is not one-size-fits-all

Historically, hospitals ordered a standard battery of pre-operative tests for every patient regardless of age or health. NICE guideline NG45 (2016) ended this practice after evidence showed that routine pre-operative testing in healthy patients rarely changed anaesthetic management and resulted in significant unnecessary cost, patient anxiety, and false-positive results requiring further investigation. Pre-operative tests are now ordered based on the specific type of surgery (grade 1–4 complexity) and the patient's comorbidities (ASA grade).

The decision tree is:

Grade 1–2 surgery (minor operations – skin lesion excision, carpal tunnel, cataract) in a healthy patient (ASA 1): Typically no blood tests required.
Grade 3 surgery (intermediate – laparoscopic cholecystectomy, inguinal hernia) or any surgery in a patient with known comorbidity: Targeted testing based on the comorbidity.
Grade 4 surgery (major – colorectal resection, hip replacement, cardiac surgery) or ASA 3–4 patients: Full pre-operative workup.

Specific tests and their pre-operative purpose

FBC (full blood count): Haemoglobin – anaemia is the most important finding; a patient with Hb below 100 g/L undergoing elective surgery should have anaemia investigated and corrected (with iron, B12, or EPO) before the operation to reduce transfusion requirements and surgical risk. Platelets – thrombocytopenia increases bleeding risk; below 80 × 10⁹/L warrants haematology review before major surgery. WBC – unexpected leucocytosis or leucopenia may indicate occult infection or haematological disorder.
Coagulation (PT/INR, APTT): Not required routinely; indicated in patients on anticoagulants (warfarin – check INR to ensure therapeutic range is appropriate), liver disease (impaired synthetic function), known bleeding disorder, or if the history suggests abnormal bleeding. Unexpected coagulopathy in a healthy patient is very rare and routine testing has low yield.
U&E (urea and electrolytes) and creatinine: Required before major surgery and in patients with CKD, diabetes, hypertension, heart failure, or on diuretics/ACEi/ARBs. Abnormal electrolytes (hypokalaemia, hyponatraemia) and renal impairment can affect anaesthetic drug choice and fluid management.
ECG (12-lead): Not a blood test, but the most commonly ordered pre-operative investigation. NICE recommends ECG for patients over 65 having major surgery, those with known or suspected heart disease, and those with unexplained breathlessness or palpitations.
Group and save / crossmatch: A blood sample typed for ABO and Rh group (group and save) is essential before any surgery with meaningful haemorrhage risk, allowing blood to be rapidly available for transfusion if required. Crossmatch (testing patient blood against specific donor units) is performed when transfusion is anticipated as likely, not just possible.