Should I run laboratory tests on this patient?

Laboratory Testing in Patients: A Clinical Decision Framework

Laboratory testing should be guided by specific clinical findings from history and physical examination rather than ordered routinely, as extensive screening panels have very low yield and generate false positives 8 times more frequently than true positives. 1

When Laboratory Testing is NOT Indicated

In alert, cooperative patients with normal vital signs and noncontributory history/physical examination presenting with psychiatric complaints, routine laboratory testing need not be performed. 1 This Level B recommendation is based on evidence showing:

• Only 4% of psychiatric ED patients required acute medical treatment within 24 hours, and history/physical examination identified 83% of these cases 1
• False positive results outnumber true positives by 8:1 in routine screening 1
• Most abnormal laboratory values found incidentally are clinically insignificant 1

High-Risk Populations Requiring Laboratory Evaluation

Certain patient groups warrant targeted laboratory testing regardless of initial presentation 1, 2:

• Elderly patients - higher risk of occult medical illness 1, 2
• First psychiatric presentation - 63% have underlying medical causes when presenting with new symptoms, though this often includes patients with abnormal vital signs or altered cognition 1
• Patients with substance abuse history 1, 2
• Lower socioeconomic status 1
• Patients with new medical complaints 2
• Abnormal vital signs (fever, tachycardia, hypertension, hypotension) - mandate metabolic workup 2
• Altered mental status, disorientation, or confusion - indicates possible delirium requiring comprehensive testing 2

Specific Clinical Scenarios Requiring Targeted Testing

New-Onset Seizures

• Serum glucose and sodium - required in all patients 3
• Pregnancy test - required for all women of childbearing age 3
• Complete metabolic panel if metabolic abnormalities suspected 3
• Calcium in patients with cancer or renal failure 3
• Magnesium in alcohol-related seizures 3

Hallucinations/Psychotic Symptoms

Core laboratory panel indicated when:

• Abnormal vital signs present 2
• Altered mental status or disorientation 2
• Fever or focal neurological deficits 2
• Patient is elderly, has no prior psychiatric history, or has substance abuse history 2

Specific tests to order:

• Serum glucose 2
• CBC if infection suspected 2
• Basic metabolic panel (electrolytes, BUN, creatinine) in patients with renal disease or medications affecting electrolytes 2
• Calcium and magnesium if hypocalcemia/hypomagnesemia suspected based on medications, renal disease, or alcoholism 2

Paresthesias

• Serum calcium (corrected for albumin) and ionized calcium 4
• Magnesium level 4
• Phosphorus and intact PTH 4
• 25-hydroxyvitamin D 4
• Avoid extensive "neuropathy panels" without specific clinical indication 4

Anorectal Abscess

• Laboratory tests should be guided by clinical findings to assess severity of illness 1
• Routine testing not needed for diagnosis 1
• Consider CBC, inflammatory markers (CRP, procalcitonin), creatinine, electrolytes, and blood gas analysis in patients with signs of systemic infection or sepsis 1
• Check serum glucose, hemoglobin A1c, and urine ketones to investigate undetected diabetes 1

Critical Pitfalls to Avoid

• Never order extensive routine panels reflexively - history and physical examination predict 83-98% of clinically significant abnormalities 2
• Do not assume psychiatric etiology without excluding medical causes in high-risk groups 2
• Avoid testing in patients with normal vital signs, normal cognition, and noncontributory examination - this generates more false positives than actionable results 1
• Remember that coagulation tests are sensitive to specimen handling and commonly lead to false-positive results if handled improperly 1
• Delay testing for bleeding disorders in patients receiving blood product transfusions until transfused clotting elements are eliminated 1

Algorithm for Decision-Making

1. Assess vital signs and mental status first 2

   • If abnormal → proceed to targeted laboratory evaluation
   • If normal → continue to step 2

2. Identify high-risk features 1, 2

   • Elderly, first presentation, substance abuse, new medical complaints
   • If present → proceed to targeted testing
   • If absent → continue to step 3

3. Complete focused history and physical examination 1

   • If findings suggest specific medical condition → order targeted tests
   • If noncontributory → laboratory testing not indicated

4. For specific symptom complexes (seizures, hallucinations, paresthesias) → follow syndrome-specific testing protocols outlined above 3, 2, 4