Blood Tests Before Surgery

Pre-operative tests at a glance

Pre-operative testing is tailored to the operation and the patient — not everyone needs every test. This table compiles the standard tests from this guide with who typically needs each one.

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.

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).²

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.

Standard pre-operative blood tests

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.

Who needs which tests?

Understanding INR before surgery

INR measures how well blood clots. For most operations, surgeons require INR <1.5.³ 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?

Pre-operative testing myths vs facts

What matters most before surgery

• Disclose everything early: every medicine, supplement and over-the-counter product — at the first surgical appointment, not the week before.
• Fix anaemia first: low haemoglobin raises transfusion risk; elective surgery is often postponed until it is corrected.
• Know your blood-thinner plan: which drug, when to stop, whether bridging is needed — agreed in advance with the surgical team.
• Diabetes control counts: poorly controlled blood sugar significantly increases surgical complications; bring it into range beforehand.

Practical notes

Hospitals usually specify which tests you need and when — often within one to two weeks of the surgery date — along with any fasting instructions in the appointment letter. Tell the team about every medicine, supplement and herbal product you take, since some affect bleeding or test results.

Bring a complete medicine list, including over-the-counter and herbal products, plus previous blood reports. Ask which abnormal results would delay surgery and who reviews them, so you know the plan if something comes back borderline.

Mildly abnormal results are common and usually lead to a review or a small correction, not a cancellation. The anaesthetist interprets your results against your overall fitness for anaesthesia, not against a perfect report.

Get the results reviewed well before the surgery date so there is time to correct anything that needs it, such as low haemoglobin. Avoid repeating tests privately without the surgical team’s knowledge — duplicate reports from different labs can confuse the record.

In India

In India, pre-surgery investigations are usually bundled into a package by the hospital, though prices vary by city and hospital; standalone tests at independent labs are often cheaper but may not match the hospital’s required panel. Confirm exactly which tests your surgical team wants before booking anything separately.

Hospitals generally prefer reports from NABL-accredited laboratories. Home sample collection is widely available in major cities if the hospital allows it, and the reference range printed on your own report is the one that counts.

Frequently Asked Questions

References

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

1. Pasternak LR. Preoperative laboratory testing: general issues and considerations. Anesthesiol Clin North America. 2004;22(1):13–25. PMID 15109688 · doi:10.1016/S0889-8537(03)00115-9
2. García-Miguel FJ, Serrano-Aguilar PG, López-Bastida J. "Preoperative assessment." Lancet. 2003;362(9397):1749–1757. PMID 14643127 · doi:10.1016/S0140-6736(03)14857-X
3. Kozek-Langenecker SA. "Perioperative coagulation monitoring." Best Pract Res Clin Anaesthesiol. 2010;24(1):27–40. PMID 20402168 · doi:10.1016/j.bpa.2009.09.009