What lab tests should be ordered for a patient presenting with nausea and vomiting?

Laboratory Tests for Nausea and Vomiting Evaluation

The most appropriate initial laboratory workup for patients presenting with nausea and vomiting should include a comprehensive metabolic panel, complete blood count, amylase, lipase, and pregnancy test for women of childbearing age.

Initial Laboratory Assessment

First-line Laboratory Tests

• Complete blood count (CBC)

  • Evaluates for infection, inflammation, anemia
  • Leukocytosis may indicate infection or inflammation
  • Anemia may suggest GI bleeding

• Comprehensive metabolic panel (CMP)

  • Electrolytes: Identifies dehydration, acid-base disturbances
  • Glucose: Hypoglycemia can cause nausea
  • BUN/creatinine: Assesses renal function and hydration status
  • Liver function tests: AST, ALT, bilirubin to evaluate for hepatitis or biliary disease 1

• Pregnancy test

  • Essential for all women of childbearing age
  • Pregnancy is one of the most common causes of nausea and vomiting 1, 2

• Pancreatic enzymes

  • Amylase and lipase to rule out pancreatitis 3

Condition-Specific Laboratory Tests

For Suspected Gastrointestinal Causes

• Thyroid function tests (TSH, free T4)

  • Hypothyroidism can present with nausea and vomiting 4

• Adrenal function tests

  • Consider cortisol level if Addison's disease is suspected 4

For Persistent or Cyclic Vomiting

• Urine toxicology screen
  • Particularly important to rule out cannabinoid hyperemesis syndrome in patients with chronic cannabis use 4

For Suspected Metabolic Causes

• Uric acid
  • Hyperuricemia may be present in certain conditions causing nausea/vomiting 5

Special Considerations

For Severe or Prolonged Symptoms

• Arterial blood gas
  • Evaluates acid-base status in severe cases with dehydration
  • Identifies metabolic alkalosis from prolonged vomiting

For Patients with Abdominal Pain

• Liver function tests
  • ALT, AST, alkaline phosphatase, bilirubin
  • Abnormalities may indicate hepatitis or biliary obstruction 1

When to Expand Testing

Second-line Laboratory Tests

• Ammonia level

  • Consider if liver disease or metabolic disorders are suspected

• Lactic acid

  • Elevated in bowel ischemia or severe sepsis

• Porphyria screening

  • For patients with recurrent unexplained symptoms and neurological manifestations 4

Common Pitfalls to Avoid

1. Relying solely on amylase for pancreatitis diagnosis

   • Lipase is more sensitive and remains elevated longer 3

2. Overlooking pregnancy testing

   • Essential in all women of childbearing age, even with seemingly obvious alternative causes 2

3. Repeated daily enzyme measurements

   • Once diagnosis is established, daily measurements of pancreatic enzymes have no value in assessing clinical progress 3

4. Missing medication-related causes

   • Review medication list thoroughly as many drugs can cause nausea and vomiting 1

Interpreting Results

• Elevated liver enzymes with hyperbilirubinemia

  • Consider hepatitis, biliary obstruction, or medication toxicity

• Elevated lipase (>3 times upper limit)

  • Highly suggestive of pancreatitis

• Electrolyte abnormalities

  • Hypokalemia, hypochloremia, and elevated bicarbonate suggest prolonged vomiting
  • Hyponatremia may indicate syndrome of inappropriate antidiuretic hormone secretion (SIADH)

Remember that laboratory testing should be guided by the clinical presentation, and not all tests are necessary for every patient. The initial evaluation should focus on identifying emergent conditions requiring immediate intervention while working toward determining the underlying cause.