Lab Tests

Laboratory Tests in Anesthesia

Anesthesiologists rely on a wide range of laboratory tests to assess patient fitness for anesthesia, guide intraoperative management, and manage postoperative care. The modern approach emphasizes targeted, indication-based testing rather than routine screening for all patients.

Here is a comprehensive outline of the lab tests commonly used in anesthesia practice, divided into logical sections.

Introduction: The Role of Laboratory Testing in Anesthesia

Laboratory tests are not ordered in isolation. They are a crucial part of the pre-anesthetic evaluation, helping to:

Identify Comorbidities: Uncover undiagnosed conditions (e.g., anemia, coagulopathy, renal disease).
Stratify Risk: Determine the patient's physiological reserve and risk of perioperative complications.
Establish a Baseline: Provide a reference point for managing significant intraoperative events (e.g., bleeding, hemodynamic shifts).
Guide Management: Direct specific anesthetic techniques, drug choices, fluid therapy, and the need for blood products.

Section 1: Hematology & Coagulation

These tests are fundamental for assessing oxygen-carrying capacity and bleeding risk, which are central to anesthetic management.

1.1. Complete Blood Count (CBC)

Hemoglobin (Hgb) & Hematocrit (Hct)
- Interpretation:
  - Low (Anemia): Reduced oxygen-carrying capacity. Causes include iron deficiency, chronic disease, or acute blood loss.
  - High (Polycythemia): Increased blood viscosity, which can impair microcirculation.
- Anesthetic Implications:
  - Anemia: Determines the threshold for blood transfusion. Severe anemia may increase the risk of myocardial ischemia. Affects the decision for anesthetic technique (e.g., regional vs. general).
  - Polycythemia: Increases risk of thrombosis. May necessitate preoperative phlebotomy in extreme cases.
Platelet Count
- Interpretation:
  - Low (Thrombocytopenia): Increased risk of bleeding. Severity is key (e.g., <100,000/mm³, <50,000/mm³).
  - High (Thrombocytosis): Can be associated with increased clotting risk.
- Anesthetic Implications:
  - Thrombocytopenia: A platelet count > 100,000/mm³ is generally considered safe for neuraxial anesthesia (spinal/epidural). Counts between 50,000-100,000 require a risk-benefit analysis. Counts < 50,000 are a contraindication to neuraxial blocks and indicate a high surgical bleeding risk. Platelet transfusion may be required.
White Blood Cell Count (WBC)
- Interpretation: Elevation suggests infection, inflammation, or stress. A very low count (leukopenia) may indicate bone marrow suppression or sepsis.
- Anesthetic Implications: A significantly elevated WBC with fever may lead to postponing elective surgery to investigate and treat infection. In sepsis, it guides aggressive hemodynamic and antibiotic management.

1.2. Coagulation Studies

Prothrombin Time (PT) & International Normalized Ratio (INR)
- Interpretation: Measures the extrinsic and common coagulation pathways. Prolonged by warfarin, vitamin K deficiency, liver disease.
- Anesthetic Implications: An INR > 1.5 is generally a contraindication to neuraxial anesthesia due to spinal hematoma risk. Guides the need for reversal (e.g., Vitamin K, FFP, PCC) before surgery.
Activated Partial Thromboplastin Time (aPTT)
- Interpretation: Measures the intrinsic and common pathways. Prolonged by heparin, hemophilia, lupus anticoagulant.
- Anesthetic Implications: Used to monitor unfractionated heparin therapy (e.g., during vascular or cardiac surgery). A significantly prolonged aPTT is a contraindication to neuraxial anesthesia.
Viscoelastic Testing (Thromboelastography - TEG / Rotational Thromboelastometry - ROTEM)
- Interpretation: Provides a real-time, functional assessment of the entire clotting process, from initiation to fibrinolysis, using whole blood. It differentiates between coagulation factor deficiency, platelet dysfunction, and fibrinolysis.
- Anesthetic Implications: The gold standard for managing massive hemorrhage (trauma, liver transplant, cardiac surgery). It allows for goal-directed, targeted blood product therapy (e.g., giving FFP for a prolonged R-time, cryoprecipitate for low functional fibrinogen, platelets for low MA/MCF), reducing unnecessary transfusions and improving outcomes.

Section 2: Metabolic & Electrolyte Panel

This panel assesses end-organ function and the body's metabolic state, which is critical for drug dosing and physiological stability.

Basic Metabolic Panel (BMP) or Comprehensive Metabolic Panel (CMP)
- Sodium (Na⁺):
  - Interpretation: Hyponatremia (<135 mEq/L) or hypernatremia (>145 mEq/L).
  - Implications: Severe or rapid shifts can cause cerebral edema or dehydration, leading to neurological changes. Crucial for managing fluid therapy, especially in TURP (Transurethral Resection of Prostate) syndrome (hyponatremia).
- Potassium (K⁺):
  - Interpretation: Hypokalemia (<3.5 mEq/L) or hyperkalemia (>5.0 mEq/L).
  - Implications: Extremely important. Hyperkalemia can cause life-threatening arrhythmias and is a major contraindication to the use of succinylcholine. Hypokalemia increases the risk of arrhythmias and potentiates the effects of non-depolarizing muscle relaxants. Severe abnormalities must be corrected preoperatively.
- Creatinine & BUN:
  - Interpretation: Markers of renal function.
  - Implications: Essential for dosing many anesthetic drugs (opioids, muscle relaxants, some antibiotics) and their metabolites that are cleared by the kidneys. Indicates the need for careful fluid management to avoid worsening renal injury.
- Glucose:
  - Interpretation: Hypoglycemia (<70 mg/dL) or hyperglycemia (>180 mg/dL).
  - Implications: Both are detrimental. Hyperglycemia is associated with worse outcomes, especially in cardiac and neurosurgery (increased risk of infection and neurological injury). Hypoglycemia can cause devastating neurological injury. Tight glycemic control is often required intraoperatively.
- Liver Function Tests (AST, ALT, Alk Phos, Bilirubin):
  - Interpretation: Indicate hepatocellular injury or cholestasis.
  - Implications: The liver is the primary site of metabolism for many drugs. Impaired liver function can prolong the duration of anesthetics and lead to a coagulopathy (decreased production of clotting factors).

Section 3: Cardiac Biomarkers

These tests are used to assess for cardiac injury and strain, particularly in patients with known cardiac disease or those undergoing high-risk surgery.

Troponin (T or I)
- Interpretation: Highly specific marker for myocardial injury. Elevation indicates a myocardial infarction (MI) or other injury (e.g., sepsis, pulmonary embolism).
- Anesthetic Implications: A preoperative troponin elevation may lead to postponing elective surgery to investigate and manage an acute coronary syndrome. Intraoperative and postoperative troponin monitoring is standard for high-risk patients to detect perioperative MI.
B-type Natriuretic Peptide (BNP or NT-proBNP)
- Interpretation: Marker of ventricular wall stress and heart failure.
- Anesthetic Implications: An elevated level is a powerful predictor of perioperative cardiac complications (heart failure, arrhythmias, MI). It guides a more conservative fluid management strategy and influences the choice of monitoring (e.g., arterial line, cardiac output monitoring).

Section 4: Intraoperative & Point-of-Care Testing (POCT)

These tests provide real-time data to guide immediate decision-making in the operating room.

Arterial Blood Gas (ABG)
- Interpretation: Provides real-time data on:
  - pH: Acidosis or Alkalosis.
  - PaCO₂: Respiratory component of acid-base balance.
  - PaO₂: Oxygenation status.
  - HCO₃⁻ & Base Excess: Metabolic component of acid-base balance.
  - Hemoglobin & Electrolytes: Often included.
- Anesthetic Implications: Indispensable for managing critically ill patients. Used to titrate ventilator settings (PaCO₂), assess oxygenation (PaO₂), diagnose and treat metabolic acidosis (e.g., in shock or massive transfusion), and monitor potassium levels.
Hemoglobin (POCT, e.g., HemoCue)
- Interpretation: Rapid measurement of Hgb.
- Anesthetic Implications: Provides a quick assessment of blood loss and helps guide the decision to transfuse red blood cells during major surgery.
Activated Clotting Time (ACT)
- Interpretation: A bedside test to measure the effect of high-dose heparin.
- Anesthetic Implications: Standard of care during cardiopulmonary bypass (CPB) and vascular procedures to ensure adequate anticoagulation and to guide protamine reversal at the end of the case.

Section 5: Blood Banking

Type and Screen
- Interpretation: Determines the patient's ABO blood type and Rh factor. Screens for the presence of common non-ABO antibodies.
- Anesthetic Implications: The minimum requirement for any surgery with a moderate to high risk of bleeding. It allows the blood bank to have compatible blood ready quickly, but not cross-matched.
Type and Crossmatch
- Interpretation: In addition to typing and screening, this test confirms that a specific unit of donor blood is compatible with the patient's red blood cells.
- Anesthetic Implications: Required for surgeries with a predictable high blood loss (e.g., major vascular, oncologic, or trauma surgery). It guarantees that fully compatible blood is available.