Routine laboratory screening tests are generally not indicated. Tests should be selected based on the patient’s medical condition and the proposed surgical procedure.

1. Hematologic studies. Hematologic studies are indicated if there are concerns regarding significant preexisting anemia, perioperative blood loss, or coagulopathy.
   1. Recent hematocrit/hemoglobin level. There is no universally accepted minimum hematocrit before anesthesia. The etiology and duration of anemia should be ascertained.
   2. Platelet studies. By history, platelet function can be assessed by easy bruising, petechiae, excessive bleeding from gums or minor cuts, and family history. Repeat complete blood count (CBC), smear, evaluation of liver function, autoimmune disease, and infectious etiology should be considered. A hematology consultation for etiology evaluation such as TTP, ITP, HIT, vWD, or malignancy may be pursued.
   3. Coagulation studies. Coagulation studies may be ordered to assess level of pharmacological anticoagulation, readiness for nerve block or neuraxial anesthesia after discontinuation of anticoagulant, synthetic liver function, and clinically relevant nutritional deficiency. Coagulation status of the patient on low-molecular-weight heparin may be monitored through measurement of anti–factor Xa levels.
   4. Blood type/antibody screen. Type and screen should be obtained if anticipating a significant intraoperative blood loss or if the patient has alloantibodies that are challenging to crossmatch.
2. Serum chemistry. A metabolic panel should be ordered in patients with risk factors for renal or hepatic disease that would affect their serum electrolytes, ability to metabolize drugs, and impact their response to fluid. It is also indicated for patients taking diuretics, digoxin, steroids, or aminoglycoside antibiotics.
   1. Hypokalemia. Low potassium levels are common in the patient receiving diuretics and are usually readily corrected by preoperative oral potassium supplementation. Mild hypokalemia (2.8-3.5 mEq/L) should not preclude elective surgery. Rapid correction with intravenous potassium may lead to dysrhythmias and cardiac arrest.
   2. Hyperkalemia. Elevated potassium is often seen in the patient with end-stage renal disease. Mild elevations in serum potassium among those with already elevated baselines is well tolerated. Treatment of hyperkalemia is warranted if concentrations exceed 6 mEq/L or if electrocardiogram (ECG) changes occur, such as peaked T waves, prolonged PR interval, P wave flattening, or widened QRS.
   3. Hyponatremia. Serum sodium levels <135 mEq/L are an independent negative prognostic indicator for perioperative outcomes. Even mild hyponatremia in healthy patients undergoing nonemergent surgery is associated with an increased incidence of 30 day mortality, coronary events, pneumonia, wound infections, and hospital length of stay. The severity of hyponatremia corresponds to worse outcomes. Low serum sodium should be considered a surrogate marker for medical comorbidities, and perhaps not itself the cause of poor outcomes. The etiology should be determined, and if possible, severe abnormalities should be corrected preoperatively.
3. ECG. The most recent ACC/AHA guidelines for patients undergoing noncardiac surgery state that a preoperative ECG is not beneficial in asymptomatic patients undergoing low-risk procedures. Preoperative ECG may be reasonable for patients with known severe arrhythmia, cerebrovascular disease, peripheral arterial disease, or structural or coronary heart disease undergoing greater than a low-risk surgical intervention. An ECG should not be done for the sole indication of advanced patient age.
4. Chest radiography. A chest x-ray (CXR) should be obtained in the patient with signs or symptoms of acute or unstable chronic cardiopulmonary disease.
5. Pulmonary function tests. Pulmonary function tests (PFTs) are used to evaluate the severity of lung disease. They have a recognized role in the evaluation of patients undergoing lung resection surgery; however, they have not been shown to be predictive of postoperative pulmonary complications in surgeries other than lung resection (see Chapters 4 and 27).