Preoperative investigation guidelines

Preoperative Investigations Guidelines 

Preoperative investigations should not be ordered as a routine battery of tests for every patient. The decision should be based on the patient’s clinical condition, comorbidities, medications, age, and the type and extent of surgery.

Two commonly referenced guidelines are:

• Indian Society of Anaesthesiologists (ISA) Practice Guidelines for Preoperative Investigations — Indian Journal of Anaesthesia (IJA), 2022
• NICE Guideline NG45: Routine Preoperative Tests for Elective Surgery, 2016

SURGERY CLASSIFICATION 

Surgery is commonly classified according to its magnitude and expected physiological stress as:

Minor → Intermediate → Major

This classification helps determine the extent of preoperative assessment and investigations. It should be considered alongside ASA physical status, patient comorbidities, and urgency of surgery.

NICE GUIDELINE 

NICE recommends a selective, risk-based approach to preoperative investigations. The need for testing is determined mainly by the ASA physical status and grade of surgery (minor, intermediate or major), along with the patient’s history and clinical findings. Routine tests are avoided when they are unlikely to influence perioperative management. For example, CBC and renal function are recommended mainly with increasing ASA status and surgical severity, while LFT, coagulation tests and CXR are generally performed only when clinically indicated.

* Renal function may be considered in selected patients at risk of perioperative AKI, including ASA III–IV undergoing minor surgery and ASA II undergoing intermediate surgery.

IJA GUIDELINE 

The IJA/ISA guideline promotes a judicious, individualised approach to preoperative investigations rather than routine testing of every patient. Investigations should be guided by the type of surgery, ASA status, physiological condition, comorbidities, medications and clinical findings, with tests ordered when they are likely to influence perioperative management. It is specifically designed for ASA I–II patients undergoing elective surgery.

* ECG age criterion applies to non-cardiac patients undergoing minor/intermediate surgery.
† CXR age criterion applies to patients ≥50 years undergoing intermediate/major surgery. 

Complete Blood Count (CBC)

CBC provides information about oxygen-carrying capacity, platelet count and the presence of leukocytosis or other haematological abnormalities.

Indications

CBC should be considered particularly for:

• Major surgery with significant blood-loss potential
• Suspected or known anaemia
• Haematological disease
• Malignancy
• Geriatric patients with suspected baseline anaemia
• Selected paediatric patients
• Situations where platelet count may influence the safety of regional/neuraxial anaesthesia

Anaesthetic relevance

CBC helps with:

Anaemia assessment → oxygen delivery assessment → transfusion planning

Platelet assessment → bleeding risk → suitability for neuraxial/regional techniques

The reference gives 100,000/µL as the usual platelet threshold below which regional blocks are avoided, while selected patients with low-grade HELLP may be considered at ≥70,000/µL.

2. Kidney Function Tests (KFT)

KFTs assess baseline renal function and help identify patients at increased risk of preoperative acute kidney injury.

Important parameters

Indications

Consider KFTs in:

• Known renal insufficiency
• Diabetes mellitus
• Hypertension
• Cardiovascular disease
• Geriatric patients
• Patients at risk of AKI
• Patients receiving potentially nephrotoxic medications
• Major surgery where significant fluid shifts are anticipated

Why potassium matters ???

Hypokalaemia

Can predispose to:

• Arrhythmias
• Muscle weakness
• Increased sensitivity to some drugs

Hyperkalaemia

Can cause:

• Conduction abnormalities
• Arrhythmias
• Potential cardiac arrest

Therefore a significant electrolyte abnormality may require correction before elective surgery, depending on severity and clinical context.

Important considerations

Renal function may be overestimated in elderly patients, because loss of muscle mass can mask a reduction in GFR despite a relatively normal serum creatinine.

The reference identifies:

• Creatinine >1.2 mg/dL → increased AKI risk
• Creatinine >2.0 mg/dL → significant cardiac risk factor
• BUN:Creatinine >15:1 → suggests prerenal dehydration/volume depletion

Pregnancy

Pregnancy is associated with increased renal blood flow and GFR. Therefore, serum creatinine is normally lower, approximately 0.5–0.6 mg/dL. A creatinine >1.0 mg/dL in pregnancy warrants evaluation.

Anaesthetic relevance

Renal assessment influences:

• Fluid management
• Electrolyte correction
• Drug dosing
• Choice and duration of neuromuscular blockers
• Risk assessment for postoperative AKI

3. Liver Function Tests (LFT)

Routine LFTs are not indicated in everyone.

LFTs are useful for assessing hepatocellular injury, hepatic synthetic function and nutritional status.

Consider LFTs in:

• Cirrhosis
• Active hepatitis
• Known liver disease
• Malnutrition
• Jaundice
• Passive hepatic congestion associated with severe right heart failure
• HELLP syndrome
• Planned major hepatic resection

Parameters may include:

• Bilirubin
• AST/ALT
• ALP
• Albumin

LFTs do not directly equal hepatic functional reserve.

For significant liver disease, also consider:

• INR
• Albumin
• Platelets
• Bilirubin
• Clinical portal hypertension
• Ascites
• Encephalopathy
• Renal function

Albumin

The reference lists normal albumin as 3.5–5.5 g/dL.

Albumin has a relatively long half-life of approximately 2–3 weeks. Therefore, it may remain normal during acute liver injury.

Albumin <2.5 g/dL suggests chronic liver disease, severe malnutrition or protein-losing states. Hypoalbuminaemia can increase the free fraction of highly protein-bound drugs.

Pseudocholinesterase

Pseudocholinesterase is synthesized in the liver and is responsible for hydrolysis of:

• Succinylcholine
• Ester local anaesthetics

Its level decreases during pregnancy and falls significantly in HELLP syndrome.

Anaesthetic relevance

Liver dysfunction may result in:

Altered drug metabolism + altered protein binding + coagulation abnormalities + prolonged drug action

Important parameters

4. Coagulation Profile

The coagulation profile evaluates secondary haemostasis and is particularly relevant when bleeding risk or neuraxial anaesthesia is a concern.

Important parameters

Indications

Consider coagulation testing in:

• Patients taking long-term anticoagulants
• Known or suspected liver disease
• Significant bleeding history
• Severe preeclampsia
• HELLP syndrome
• Patients in whom regional/neuraxial anaesthesia is being considered

PT / INR

PT is particularly sensitive to abnormalities in hepatic synthetic function because Factor VII has a short half-life.

The reference identifies PT prolongation >3 seconds / INR >1.5 as suggestive of severe hepatic synthetic dysfunction.

Fibrinogen

Normal fibrinogen is approximately 200–400 mg/dL.

In pregnancy, fibrinogen physiologically increases to approximately 400–500 mg/dL. A significant fall, particularly around delivery, can be an early warning sign of severe postpartum haemorrhage.

Anaesthetic relevance

Coagulation assessment helps determine:

Bleeding risk → need for correction/blood products → safety of neuraxial anaesthesia

5. 12-Lead ECG

A 12-lead ECG provides information regarding:

• Heart rate
• Rhythm
• Conduction
• Ischaemic changes
• Evidence of LV hypertrophy

Indications

Consider ECG in patients with:

• Known coronary artery disease
• Previous myocardial infarction
• Heart failure
• Hypertension
• Renal impairment
• Relevant cardiovascular symptoms
• Patients receiving drugs such as digoxin or diuretics
• Selected older/high-risk patients

Anaesthetic relevance

ECG may identify:

Silent ischaemia + Arrhythmias + Conduction abnormalities + LVH

These findings may alter the need for further cardiovascular evaluation, monitoring and perioperative planning.

6. Pulmonary Investigations

Spirometry / Pulmonary Function Tests

Important parameters include:

• FEV₁
• FVC
• DLCO

Indications

Consider in:

• Known COPD
• Known asthma when baseline assessment is useful
• Major thoracic surgery
• Planned lung resection

For lung resection, the reference highlights predicted postoperative FEV₁ and DLCO >40% as an important threshold.

Spirometry and chest radiography should not be used routinely in asymptomatic patients simply to predict postoperative pulmonary complications.

7. Chest X-ray

Chest radiography is not a routine investigation for every patient.

It may be considered when there is:

• Significant respiratory disease
• Significant cardiac disease
• Relevant respiratory symptoms
• Suspected pulmonary pathology
• A specific surgical indication

Potential findings include:

• Hyperinflation
• Cardiomegaly
• Pulmonary pathology

8. Arterial Blood Gas (ABG)

ABG provides information about:

• pH
• PaO₂
• PaCO₂
• HCO₃⁻

Indications

Consider ABG in:

• Severe asthma/COPD undergoing major thoracic or abdominal surgery
• Significant respiratory disease with concern regarding gas exchange
• Kidney failure with tachypnoea
• Suspected significant acid-base disturbance

Anaesthetic relevance

ABG establishes a baseline for:

Oxygenation + Ventilation + Acid-base status

9. Echocardiography

This is not a routine preoperative test.

Consider when there is clinical suspicion of significant structural/functional heart disease.

Examples:

• Suspected heart failure

Symptoms:

• Dyspnoea
• Orthopnoea
• PND
• Peripheral oedema

– Suspected significant valvular disease

For example:

• Severe AS
• Significant MR

– Unexplained dyspnoea

Known cardiac disease with a clinically important change

Why does echo matter to the anaesthesiologist?

It gives information about:

• LV systolic function
• Diastolic function
• Valve pathology
• Chamber size
• Pulmonary pressures
• RV function

This can fundamentally change:

Anaesthetic technique + monitoring + haemodynamic goals.

What is truly “routine”?

There is no universal routine investigation panel.

For a young, healthy, asymptomatic patient undergoing minor surgery, extensive laboratory and imaging investigations may not be required.

For a patient with significant comorbidities or major surgery, investigations become more extensive and should be targeted to the clinical problem.

Every investigation should have a reason. Ask: “If this result is abnormal, will it change my management?”

References

1. Miller RD, Cohen NH, Eriksson LI, et al. Miller’s Anesthesia. Elsevier.
   — Perioperative evaluation, preoperative testing, and anesthetic risk assessment.
2. Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail’s Clinical Anesthesiology. McGraw-Hill.
   — Preoperative evaluation and selection/interpretation of laboratory investigations.
3. Barash PG, Cullen BF, Stoelting RK, et al. Clinical Anesthesia. Wolters Kluwer.
   — Preoperative assessment and perioperative laboratory testing.
4. American Society of Anesthesiologists (ASA). Practice Advisory for Preanesthesia Evaluation.
   — Guidance on appropriate preanesthetic assessment and investigations.
5. National Institute for Health and Care Excellence (NICE). Routine Preoperative Tests for Elective Surgery (NG45).
   — Evidence-based recommendations on when CBC, coagulation studies, renal function, ECG, chest X-ray and other investigations should be performed.

6.    Goneppanavar U et al. Preoperative Investigations: Practice             Guidelines from the Indian Society of Anaesthesiologists. Indian J Anaesth. 2022;66(5):319–343, Table 6, Recommendations 1–10.