When do you order laboratory tests on a patient?

When to Order Laboratory Tests on a Patient

Laboratory tests should be ordered based on clinical suspicion from history and physical examination findings, not as routine screening for all patients. 1 Targeted testing guided by specific symptoms, risk factors, and clinical presentation is more effective and cost-efficient than broad, untargeted testing.

General Principles for Laboratory Test Ordering

• Order tests only when they will change management decisions - Tests should have reasonable diagnostic yield, be low risk, reasonable in cost, and improve patient management 1
• Avoid "shotgun" approach - Deliberate, personalized testing is preferred over broad-based testing 1
• Consider pre-test probability - Higher pre-test probability increases yield of positive results 2
• Document the purpose - Tests should be ordered with a specific diagnostic, prognostic, or treatment goal in mind 3

Specific Clinical Scenarios Warranting Laboratory Testing

1. Suspected Infection

• Order CBC with differential when infection is suspected within 12-24 hours of symptom onset 1
• Consider elevated WBC count (>14,000 cells/mm³) or left shift (bands >16% or >1500 cells/mm³) as warranting careful assessment for bacterial infection 1
• For fever in long-term care facility residents, obtain targeted tests based on clinical presentation rather than routine panels 1

2. Psychiatric Presentations

• For patients with acute psychosis:
  • Initial laboratory evaluation should include CBC, basic metabolic panel, thyroid function tests, urinalysis, and urine toxicology screen 4
  • More extensive testing for patients with abnormal vital signs, disorientation, or first-episode psychosis 4
  • Patients over 65 years require more comprehensive evaluation including B12 levels 4
  • Avoid routine extensive testing in psychiatric patients with normal vital signs and no concerning history 1

3. Procedural Sedation

• Review previous medical records and interview patient/family to identify abnormalities of major organ systems
• Conduct focused physical examination (vital signs, auscultation, airway evaluation)
• Review available laboratory test results
• Order additional tests only when guided by patient's medical condition, physical examination, and likelihood results will affect sedation management 1

4. Suspected Cardiac Issues

• For patients with syncope, cardiac evaluation (including laboratory tests) is recommended when cardiac syncope is suspected based on clinical features 1
• During COVID-19 pandemic, prioritize cardiac testing based on urgency of management decisions 1

5. Neurological Evaluation

• For suspected Alzheimer's disease, laboratory testing should be guided by the patient's individual medical, neuropsychiatric, and risk profile when diagnostic uncertainty remains 1
• Use a tiered approach to testing, starting with more common conditions before testing for rare disorders 1

Factors That Increase Need for Laboratory Testing

• Abnormal vital signs - Fever, tachycardia, hypotension, tachypnea 1, 4
• Altered mental status - Confusion, disorientation, delirium 1, 4
• Age extremes - Elderly patients and very young patients 4
• Multiple comorbidities - Especially those affecting major organ systems 4
• Medication use - Particularly those requiring therapeutic monitoring 4
• First presentation of symptoms - First episodes often warrant more thorough evaluation 4

Common Pitfalls to Avoid

• Overreliance on routine testing - Less than 1 test in 50 ordered routinely yields clinically meaningful findings 4
• Attributing symptoms to psychiatric causes without adequate medical workup 4
• Unnecessary delays in treatment due to excessive testing 4
• Ignoring false positives - Routine testing in low-risk patients increases false positive rates 2
• Misinterpreting results - Laboratory abnormalities must be interpreted in clinical context 5

Best Practices for Test Ordering

• Document the purpose of each test (diagnosis, prognosis, treatment monitoring, or screening) 3
• Consider patient characteristics including age and comorbidities when ordering tests 3
• Utilize decision support tools when available to guide appropriate test selection 6
• Consult with laboratory professionals for guidance on test selection and interpretation 7
• Review test results promptly and adjust treatment plans accordingly

By following these principles, clinicians can ensure laboratory testing is used appropriately to improve patient outcomes while minimizing unnecessary costs and potential harms from overtesting.