What lab work should be ordered for a patient presenting with hypotension?

Laboratory Workup for Hypotension

For patients presenting with hypotension, a comprehensive laboratory evaluation should include complete blood count, urinalysis, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, glucose, lipid profile, liver function tests, iron studies, and thyroid-stimulating hormone to optimize management and identify the underlying cause. 1

Initial Laboratory Tests

Essential Tests

• Complete blood count (CBC)
• Urinalysis
• Serum electrolytes (sodium, potassium, calcium, magnesium)
• Blood urea nitrogen (BUN)
• Serum creatinine
• Glucose (fasting blood glucose or glycohemoglobin)
• Lipid profile
• Liver function tests
• Thyroid-stimulating hormone (TSH)
• Iron studies (serum iron, ferritin, transferrin saturation)

Additional Tests Based on Clinical Suspicion

• B-type natriuretic peptide (BNP) or N-terminal pro-B-type natriuretic peptide (NT-proBNP) - particularly useful when heart failure is suspected 1
• Intact parathyroid hormone (iPTH) - if hypercalcemia is present 2
• Cortisol levels and ACTH stimulation test - to rule out adrenal insufficiency 3

Diagnostic Imaging and Other Tests

• 12-lead electrocardiogram - should be performed at the initial encounter 1
• Chest radiograph (PA and lateral) - to assess for pulmonary congestion or other cardiopulmonary pathology 1
• Echocardiography - if cardiac etiology is suspected, to assess ventricular function, structural abnormalities, and valvular function 1

Specialized Testing Based on Clinical Context

For Suspected Cardiogenic Causes

• Cardiac biomarkers (troponin, CK-MB) - to rule out myocardial injury
• BNP or NT-proBNP - elevated levels support a diagnosis of heart failure 1
• Echocardiography - to assess left ventricular ejection fraction (LVEF), wall motion, and structural abnormalities 1

For Suspected Hypovolemic Causes

• Hematocrit - note that a single measurement has low sensitivity for detecting bleeding 1
• Serial measurements of vital signs and hematocrit - more valuable than single readings
• Lactate levels - to assess tissue perfusion

For Suspected Endocrine Causes

• Adrenal function tests (cortisol, ACTH stimulation)
• Aldosterone and renin levels - particularly in cases with electrolyte abnormalities 3
• Thyroid function tests - hypothyroidism can cause hypotension

For Suspected Infectious Causes

• Blood cultures
• Lactate levels
• Procalcitonin (if sepsis is suspected)

Interpretation Considerations

• A single hematocrit measurement should not be used as an isolated marker for bleeding, as it has low sensitivity (0.5) for detecting traumatic hemorrhage requiring intervention 1
• Hyponatremia often accompanies hypotension and may require specific evaluation and management 4
• BNP levels can be affected by age, sex, weight, and renal function, and should be interpreted cautiously 1

Common Pitfalls to Avoid

• Relying solely on a single laboratory value to guide management
• Failing to repeat critical laboratory tests when clinical status changes
• Not considering medication-induced hypotension (e.g., ACE inhibitors, especially in specific populations like tetraplegic patients) 5
• Overlooking endocrine causes of hypotension, which may require specific laboratory evaluation 3
• Delaying treatment while waiting for laboratory results in severely hypotensive patients

By systematically evaluating these laboratory parameters, clinicians can identify the underlying cause of hypotension and implement appropriate management strategies to improve patient outcomes.