Preoperative Laboratory Testing

Preoperative Laboratory Testing: General Principles - Lab Logic Intro

• Core Aim: Optimize patient safety, not routine screening. Tests guide management.
• Rationale for Testing:
  • Detect significant abnormalities impacting perioperative care.
  • Assess severity/stability of known co-morbidities.
  • Establish crucial baseline values.
• Key Considerations for Ordering:
  • Patient's clinical status (history, physical exam).
  • Type, invasiveness, and urgency of surgery.
  • Potential for significant blood loss.
  • Medications (e.g., anticoagulants, diuretics).
• Avoid: Indiscriminate "shotgun" testing; order only if results will influence decisions.
• Risks of Over-testing: False positives, increased costs, unnecessary delays, patient anxiety.

⭐ Routine preoperative testing in healthy individuals undergoing low-risk surgery is generally not beneficial and may lead to a cascade of further unnecessary investigations.

Preoperative Laboratory Testing: Common Tests & Indications - Test Lineup

• CBC (Hb, Plt)
  • Indications: Anemia, infection, bleeding risk, major blood loss anticipated, chronic illness (renal, cardiac, liver).
  • Thresholds: Hb < 7-8 g/dL; Platelets < 50,000/µL (significant risk).
• Coagulation Profile (PT/INR, aPTT)
  • Indications: Bleeding history, anticoagulant therapy (e.g., warfarin), known liver disease, major surgery with high bleeding risk.
  • INR target: e.g., 2.0-3.0 for warfarin.
• Serum Electrolytes & Renal Function (K+, Creatinine)
  • Indications: Diuretic use, renal disease, endocrine disorders (e.g., Diabetes Mellitus), dehydration, elderly patients, major surgery.
  • K+ alert: < 3.0 mEq/L or > 5.5 mEq/L.
• Blood Glucose
  • Indications: Diabetes (known or suspected), corticosteroid use, obesity, elderly.
  • Perioperative target: < 180 mg/dL.
• Liver Function Tests (LFTs)
  • Indications: Known liver disease, history of alcohol abuse, use of hepatotoxic drugs, jaundice.
• Urinalysis
  • Indications: Symptoms of UTI, urologic surgery, suspected renal pathology.
• ECG (Electrocardiogram)
  • Indications: Age (e.g., > 50-65 years), known cardiac or significant respiratory disease, hypertension, diabetes, major surgery.
• Chest X-Ray (CXR)
  • Indications: Age (e.g., > 60-75 years), known cardiopulmonary disease, new respiratory symptoms, thoracic surgery. (Not routine if asymptomatic).
• β-hCG (Pregnancy Test)
  • Indications: All women of childbearing potential before procedures that could affect a fetus.

⭐ Routine preoperative testing in asymptomatic, healthy patients undergoing minor or intermediate-risk non-cardiac surgery is generally NOT recommended due to low diagnostic yield. (Ref: ASA, Choosing Wisely)

Preoperative Laboratory Testing: Special Populations & Conditions - Focus Group Labs

• Focus labs on H&P findings & specific co-morbidities; avoid routine batteries.
• Renal (CKD): Serum Creatinine, eGFR, Electrolytes (K+), CBC (anemia).
• Hepatic (CLD): LFTs, Albumin, PT/INR, Platelet count (thrombocytopenia).
• Cardiac (CVD): Baseline ECG, K+. Consider Troponin (acute symptoms), BNP (heart failure).
• Pulmonary (Severe COPD/Asthma): ABG (if severe/acute exacerbation), CXR.
• Diabetes Mellitus: HbA1c, Blood glucose.
• Thyroid Disease: TSH.
• Anticoagulation/Bleeding Hx: PT/INR, aPTT, Platelets.
• Pregnancy: CBC, Blood group & Rh screen, Urinalysis.

⭐ ACEi/ARB users: Check baseline Creatinine & K+ due to hyperkalemia risk, esp. with diuretics.

Preoperative Laboratory Testing: Interpretation & Management - Decision Drivers

• Core Principle: Balance surgical need against risk from abnormality.
• Key Decision Triggers:
  • Hb < 7-8 g/dL: Transfuse if symptomatic/major surgery.
  • K+ < 3.0 or > 5.5 mEq/L: Correct/postpone elective; cardiac risk.
  • INR > 1.5 (no anticoagulants): Investigate/correct.
  • Platelets < 50,000/μL: Transfuse for major surgery.
  • Glucose > 180 mg/dL: Optimize; infection risk.
• Management Strategy: Correct if possible, delay elective if severe, or proceed with caution if urgent.

⭐ Severe asymptomatic hypokalemia (K+ < 2.5 mEq/L) often warrants postponement of elective surgery due to arrhythmia risk.

High-Yield Points - ⚡ Biggest Takeaways

• Routine testing is not indicated for healthy patients, minor surgery.
• Tests guided by history, exam, and surgical invasiveness.
• Hb/Hct for major surgery with expected blood loss or anemia.
• Creatinine/BUN if >50 yrs, renal disease, diabetes, hypertension.
• ECG if >50 yrs, cardiac disease, or significant risk factors.
• Urine Pregnancy Test (UPT) for all women of childbearing age.
• Coagulation studies for bleeding history or anticoagulant use.