What baseline laboratory tests should be ordered within the first 24 hours after coronary artery bypass grafting?

Baseline Laboratory Tests Within 24 Hours Post-CABG

All patients should have cardiac biomarkers (CK-MB or troponin), complete blood count, basic metabolic panel, and coagulation studies obtained within the first 24 hours after CABG to detect procedural myocardial infarction, bleeding complications, electrolyte disturbances, and renal dysfunction.

Essential Laboratory Monitoring

Cardiac Biomarkers for Procedural MI Detection

• Obtain CK-MB or standard (non-hs) troponin at baseline (preoperatively) and within 48 hours post-procedure to identify clinically significant procedural myocardial infarction 1
• For patients with normal baseline biomarkers, procedural MI is defined as CK-MB ≥10× upper reference limit (URL) OR ≥5× URL with new pathological Q waves 1
• Standard troponin elevation ≥70× URL OR ≥35× URL with ECG changes indicates procedural MI 1
• Obtain at least 2 postprocedure samples measured 8 hours apart, beginning 3-6 hours after surgery; if values are elevated, continue draws until peak is reached 1

Hematologic Parameters

• Complete blood count is essential as platelet counts typically increase above reference range in the postoperative period, peaking well above normal by day 14 2
• White blood cell counts rise above reference range, primarily due to increased neutrophils 2
• Preoperative platelet count ≤233 × 10³/µl predicts increased postoperative bleeding risk (P < 0.001) 3
• Hemoglobin and hematocrit monitoring identifies bleeding complications requiring intervention 2

Coagulation Studies

• Obtain PT/INR and aPTT as mean postoperative coagulation values typically remain within reference ranges at extubation but require monitoring 2
• **Preoperative fibrinogen <3.1 g/l is associated with excessive bleeding** (>500 ml in first 24 hours) and should be checked 3
• This fibrinogen threshold has significant predictive value (P = 0.01) for postoperative hemorrhage 3

Metabolic and Renal Function

• Basic metabolic panel is mandatory as most patients develop persistent postoperative hyperglycemia requiring management 2
• Electrolyte abnormalities (particularly potassium and magnesium) must be identified and corrected to prevent arrhythmias 2
• Estimated glomerular filtration rate ≤90.8 ml/min predicts increased bleeding risk (P < 0.001) and identifies renal dysfunction 3
• Creatinine monitoring detects acute kidney injury from cardiopulmonary bypass 2

Hepatic and Myocardial Enzymes

• Creatine kinase levels rise to nearly 4 times the upper limit of reference range in early postoperative period after uncomplicated CABG 2
• Lactate dehydrogenase, AST, and ALT increase above reference range postoperatively 2
• Total protein and albumin values fall below reference range throughout the postoperative period 2

Critical Monitoring Beyond Laboratory Tests

Electrocardiographic Surveillance

• Continuous ECG monitoring for arrhythmias must be performed for at least 48 hours after CABG (Class I recommendation) 1
• This detects postoperative atrial fibrillation, which most commonly occurs between days 2-4 post-surgery 1
• Obtain baseline ECG and repeat within 24 hours to identify new Q waves or conduction abnormalities that indicate procedural MI 1

Hemodynamic Monitoring Considerations

• Pulmonary artery catheterization is indicated in patients with cardiogenic shock (Class I) 1
• PAC placement can be useful in patients with acute hemodynamic instability in the early postoperative period (Class IIa) 1

Common Pitfalls and Clinical Pearls

Interpretation Challenges

• Recognize that laboratory values change substantially after uncomplicated CABG - elevated CK, LDH, AST, ALT, and WBC count are expected and do not necessarily indicate complications 2
• Do not use high-sensitivity troponins for procedural MI diagnosis as they inflate MI rates due to ≥10-fold lower URL compared to standard assays 1
• Hyperglycemia is nearly universal and requires active management with insulin to maintain glucose ≤180 mg/dL 4

Risk Stratification

• Male gender, BMI ≤28.3 kg/m², platelet count ≤233 × 10³/µl, eGFR ≤90.8 ml/min, and fibrinogen ≤3.1 g/l are independent predictors of excessive postoperative bleeding 3
• These preoperative values help identify high-risk patients requiring more intensive monitoring 3

Timing Considerations

• Most major complications occur during the first 6 hours after CABG, with risk declining thereafter 1
• Laboratory abnormalities peak at different times: CK-MB and troponin peak within 6 hours, while platelet counts peak around day 14 2, 5
• Cardiovascular performance differences between on-pump and off-pump CABG are eliminated by 24 hours 5