01Principles and purposeThe professional or clinical skill and the decisions it supports.
Preoperative tests can establish a baseline, reveal a modifiable problem or answer a specific concern that changes anaesthesia or surgery. Testing has costs beyond the blood draw: incidental abnormalities create work, delay and anxiety, while indiscriminate testing can encourage clinicians to overlook a more informative history. A useful request therefore anticipates what a normal or abnormal result would change. The same laboratory value can carry different implications before a minor procedure and before a major operation with expected blood loss.
NICE NG45 addresses routine testing for elective surgery. It is not a restriction on investigations needed for active bleeding, infection, kidney injury or another acute presentation. Interpret results through three questions: is the result reliable, is the abnormality new, and what consequence does it have for the proposed care? The final output should be a documented decision with responsibility and timing. Merely forwarding a result to an inbox or writing to repeat later leaves the perioperative risk unresolved.
Key points
- 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.
- Compare renal and electrolyte results with baseline, medicines, intake and losses before attributing every abnormality to chronic disease.
- Normal routine clotting results cannot reliably exclude a clinically relevant direct oral anticoagulant effect.
- Allocate responsibility for each abnormal result, the action it requires and the decision about proceeding, optimisation or postponement.
02Situations and prioritiesThe context, relevant information and actions that matter most.
Establish the planned procedure, its likely physiological burden and blood loss, and the patient’s comorbidities and functional limitations. ASA status is relevant to the routine-testing matrix but does not replace a clinical history. A healthy person having minor surgery has different testing needs from someone with renal disease having a complex abdominal operation.
Assess haemoglobin alongside prior results, MCV, symptoms, bleeding history and the likely cause. Microcytosis can support iron deficiency but is not a diagnosis by itself; inflammation, mixed deficiencies and haemoglobin disorders may complicate the picture. The purpose of recognising anaemia is to identify its cause and an appropriate plan before expected surgical blood loss, not to issue a generic clearance label.
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.
Ask about bleeding with previous surgery or dentistry, spontaneous bleeding, liver disease and family history, and identify anticoagulants or antiplatelet medicines with their last doses. Routine coagulation results assess selected pathways and are not a universal bleeding-risk screen. A platelet count measures number rather than all aspects of platelet function.
Check identity, sampling time, units, haemolysis comments and whether sampling from an infusion line may have affected the result. A surprising value requires clinical correlation and sometimes repeat sampling. Suspected artefact is a reason to verify promptly, not to ignore a dangerous result when the patient’s symptoms or ECG support it.
03Assessment and interpretationHow to gather information, assess the situation and recognise uncertainty.
Consider the information, its meaning and its limitations before deciding what follows.
- 01
Full blood count and anaemia studies - Why
- Identify anaemia, platelet abnormalities and relevant baseline haematological information.
- Interpretation and limitations
- 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.
- 02
Creatinine, electrolytes and glucose assessment - Why
- Assess kidney function, metabolic abnormalities and safety implications for perioperative care.
- Interpretation and limitations
- Use the NG45 surgery/ASA matrix and individual AKI risk to decide routine renal testing. Abnormal results can alter fluid, medicine and anaesthetic planning. HbA1c reflects longer-term glycaemia rather than current glucose; for people with diabetes, NG45 recommends testing if no result is available from the preceding three months.
- 03
Haemostasis tests when indicated - Why
- Clarify selected clotting abnormalities when the history or clinical context makes the result actionable.
- Interpretation and limitations
- NG45 advises against routine haemostasis testing and supports consideration in chronic liver disease for intermediate or major surgery. Normal INR or APTT does not reliably measure all direct oral anticoagulant effects. Establish the drug, dose timing, renal function and procedure bleeding risk, and seek an individual perioperative anticoagulation plan rather than assuming a normal panel permits surgery.
- 04
Blood-bank testing and other targeted requests - Why
- Prepare for plausible transfusion needs and investigate specific abnormalities beyond a routine panel.
- Interpretation and limitations
- The requirement for group-and-screen or crossmatched blood depends on procedure, bleeding risk and the institution’s blood-ordering arrangements. These requests are distinct from measuring haemoglobin. Liver tests, inflammatory markers, pregnancy testing or other investigations should answer a defined question; local sample-validity and identification requirements must be checked with the receiving service.
04Worked approachesCases with ordered reasoning, an action and a check of the outcome.
01Worked case: anaemia before major surgeryTurn an abnormal result into a planA 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.+
- 1Confirm 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.
- 2Review 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.
- 3Discuss 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.
- 4The 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.
- 5Verify 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.
02Reliable interpretationHandle an unexpected abnormal resultA preoperative blood test reports a substantial new biochemical abnormality that does not fit the referral history.+
- 1Assess the patient and establish whether symptoms, observations or an ECG imply immediate danger. Check sample identity and quality, previous values, recent medicines and fluid losses.
- 2Repeat or extend testing promptly when appropriate, while treating a clinically supported emergency rather than waiting for confirmation of every detail. Discuss the finding with the relevant senior clinician.
- 3Document the likely explanation, the action taken and the implications for the procedure. Communicate directly when delay or failure to act could affect safety; an electronic flag alone is insufficient.
03Proportionate testingChoose a focused elective requestAn asymptomatic patient is being assessed for elective surgery and has no new clinical concern requiring diagnostic testing.+
- 1Identify the procedure category and ASA status, review available recent results and use NG45 to select routine tests. Avoid repeating a test merely because it was performed elsewhere.
- 2Add a targeted investigation only when the history, medicines or examination provides a reason and the result can affect care. Explain the purpose to the patient and obtain appropriate consent.
- 3Arrange a mechanism to review and act on the results before the procedure. If testing does not change the anticipated management, reconsider whether it is necessary.
05Feedback, follow-up and evidenceReview outcomes, seek feedback and identify what to improve.
- For treated anaemia, specify a review point that can inform surgical planning and verify response rather than assuming the prescription produced correction.
- After addressing dehydration or a medicine-related electrolyte problem, repeat assessment and testing at a clinically appropriate interval before concluding that the risk has resolved.
- Keep the anticoagulation plan accessible to the patient, surgical team and anaesthetist, including interruption and resumption decisions where required.
- Check that primary-care and hospital test results are reconciled so the same unresolved abnormality is not repeatedly rediscovered without ownership.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
Reference ranges are population descriptions
A result can be within a reference interval yet represent a clinically important change for one patient. Conversely, a small stable deviation may have little effect on a low-risk procedure. Use both the individual baseline and the consequence for care rather than responding to the colour of a laboratory flag.
Albumin is not a nutrition score
Low albumin may reflect inflammation, liver dysfunction, renal or gastrointestinal loss and fluid distribution as well as nutritional problems. It can signal increased vulnerability without identifying one correct treatment. Assess nutritional intake and clinical context rather than prescribing a solution solely to normalise albumin.
Risk information must reach a decision maker
An abnormal test becomes useful only when someone interprets it and alters or confirms the plan. Trace the pathway from sample to result to discussion to action. In a handover, include the unresolved decision and its deadline rather than saying bloods abnormal.
Explaining uncertainty to the patient
A clear explanation distinguishes an unexpected finding, its possible causes and what additional assessment will establish. Avoid telling someone that surgery is unsafe before the responsible team has weighed the urgency, alternatives and modifiable risks with them.
07Common pitfallsFrequent interpretation and management errors.
- 01
Ordering the same preoperative panel for every procedure ignores the different benefits and harms of testing.
- 02
Calling all microcytic anaemia iron deficiency without examining the iron studies and clinical context can misdirect treatment.
- 03
Using normal INR or APTT as evidence that a direct oral anticoagulant has no clinically relevant effect is unreliable.
- 04
Declaring an operation cleared because blood results are normal neglects symptoms, functional capacity, examination and the nature of surgery.