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

Initial Laboratory Evaluation for Nausea and Vomiting

Order a complete blood count, basic metabolic panel (electrolytes, glucose, BUN, creatinine), liver function tests (AST, ALT, bilirubin), serum lipase, and urinalysis as your first-line laboratory workup for any patient presenting with nausea and vomiting. 1

Core Laboratory Panel

The initial laboratory assessment should systematically evaluate for the most common and dangerous underlying causes:

• Complete blood count (CBC) detects infection (elevated WBC), anemia (from occult bleeding or chronic disease), or hematologic abnormalities that may underlie the symptoms 2, 1

• Basic metabolic panel is essential to identify:

  • Hypokalemia, hypochloremia, and metabolic alkalosis resulting from prolonged vomiting 1
  • Hypoglycemia or hyperglycemia, including diabetic ketoacidosis 1
  • Dehydration and renal dysfunction through BUN and creatinine 3, 1

• Liver function tests (AST, ALT, bilirubin) exclude hepatobiliary pathology as a cause 2, 1

• Serum lipase screens for acute pancreatitis, which commonly presents with nausea and vomiting 3, 2, 1

• Urinalysis evaluates hydration status, detects urinary tract infection, identifies ketonuria, and excludes renal disease 3, 2, 1

Context-Specific Additional Testing

Beyond the core panel, tailor additional tests based on clinical presentation:

• Urine pregnancy test in all women of childbearing age, as pregnancy is a common cause that must not be missed 4

• Thyroid-stimulating hormone (TSH) when hypothyroidism is suspected based on clinical features 3, 2, 1

• Urine drug screen in adolescents and young adults to detect cannabis use (Cannabis Hyperemesis Syndrome) or other substances provoking vomiting 1

• Serum calcium when hypercalcemia is suspected, as elevated calcium directly causes nausea and vomiting 1

• Morning cortisol or ACTH stimulation test if Addison's disease is clinically suspected based on hypotension, hyperpigmentation, or unexplained hyponatremia 3, 1

• COVID-19 testing in settings with high community prevalence, as gastrointestinal symptoms may precede respiratory manifestations 1

Critical Clinical Pitfalls

Avoid these common errors that lead to missed diagnoses or complications:

• Do not delay imaging when bilious vomiting is present—this may indicate a surgical emergency such as malrotation with volvulus requiring immediate intervention 1

• Aggressively correct electrolyte disturbances, particularly hypokalemia and hypomagnesemia, to prevent worsening symptoms and cardiac complications 1

• Monitor QTc interval when administering ondansetron to patients with electrolyte abnormalities due to risk of drug-induced arrhythmia 1

• Obtain a non-judgmental substance-use history to identify Cannabis Hyperemesis Syndrome, which is increasingly common; avoid stigmatizing patients 1

• Check medication levels of digoxin, phenytoin, carbamazepine, and tricyclic antidepressants if these drugs are being used, as they commonly cause nausea at toxic levels 3

When Initial Labs Are Non-Diagnostic

If symptoms persist beyond 7 days or worsen despite normal initial laboratory results:

• Upper endoscopy (EGD) is recommended to rule out mechanical obstruction, peptic ulcer disease, or malignancy 1

• Gastric emptying scintigraphy should be considered when gastroparesis is suspected based on postprandial symptoms and early satiety 2, 1

• Head CT scan is indicated if neurological signs suggest an intracranial process as the underlying cause 1

The key is recognizing that while acute self-limited gastroenteritis is the most common cause of nausea and vomiting 4, the laboratory workup must systematically exclude life-threatening metabolic, infectious, and obstructive causes before attributing symptoms to benign etiologies.