Synopsis
Select and interpret preoperative blood tests in the context of the proposed operation, comorbidity and clinical trajectory, then translate abnormal findings into a proportionate investigation and perioperative plan.
- Order a test because the surgical context or a clinical question makes the result useful, not because a standard panel exists.
- Use the NG45 elective-testing recommendations with both surgery grade and ASA status; acute illness requires a separate diagnostic judgement.
- Interpret haemoglobin with its trend and red-cell indices, and investigate the cause of anaemia rather than simply labelling the patient unfit.
Key red flags
A major new electrolyte abnormality, suspected active bleeding or acute deterioration must be clinically assessed promptly rather than filed as an incidental preoperative result.
A result obtained during bowel preparation, vomiting, diuretic treatment or poor intake may reveal a reversible problem. Compare with baseline and examine the patient. Unexpected substantial potassium disturbance needs prompt verification and assessment, including an ECG when clinically indicated; a potentially dangerous abnormality cannot wait merely because the sample was labelled preoperative.
Reasoning priorities
Identify anaemia, platelet abnormalities and relevant baseline haematological information.
NG45 recommends a full blood count for major or complex elective surgery across ASA grades; it does not recommend it routinely for every minor or intermediate procedure. When anaemia is present, select ferritin, transferrin saturation and other studies according to the clinical pattern. CPOC’s 2025 anaemia guidance emphasises investigation and management across the perioperative pathway.
Worked reasoning
A 58-year-old awaiting elective major bowel surgery has haemoglobin 94 g/L, MCV 72 fL and ferritin 10 micrograms/L. They are not bleeding actively and have stable observations. Surgery is scheduled in four weeks.
- Confirm the result and compare with earlier counts. The small red cells and low ferritin strongly support iron deficiency, but the clinical task also includes establishing why iron is deficient and whether there are symptoms or additional contributors.
- Review the bowel diagnosis, bleeding history, nutrition, medicines and available investigations. Coordinate with the surgical and anaesthetic teams and the perioperative anaemia pathway, rather than treating the laboratory finding as an isolated clerical obstacle.
- Discuss an appropriate iron-replacement and optimisation strategy according to cause, tolerance, urgency and available time, with investigation of the source in parallel. Do not assume every low haemoglobin requires transfusion or that a laboratory target alone determines benefit.
- The decision is a named optimisation and review plan before the operation, with the timing of surgery considered jointly in light of its urgency and the patient’s priorities. No universal haemoglobin cancellation threshold can replace that assessment.
- Verify who will arrange treatment, when the blood count and response will be reviewed, and how the result will reach the team making the proceed-or-defer decision. Check that the cause of iron deficiency has not been lost during correction of the number.