What laboratory tests should be ordered in a clinical setting?

Laboratory Tests in Clinical Settings: A Comprehensive Approach

Laboratory tests should be ordered based on specific clinical questions rather than routine intervals, with selection guided by patient history, physical examination findings, and suspected diagnoses.

Core Laboratory Tests for Initial Evaluation

• Complete blood count (CBC) with differential, including hemoglobin, hematocrit, and platelet count, serves as a fundamental assessment tool for various conditions 1
• Basic metabolic panel, including electrolytes (sodium, potassium, calcium, magnesium), renal function tests (BUN, creatinine), and glucose, provides essential information about metabolic status 1
• Liver function tests, including transaminases and bilirubin, are important for assessing hepatic function 1
• Coagulation studies, including prothrombin time (PT), international normalized ratio (INR), and activated partial thromboplastin time (aPTT), are essential for evaluating bleeding risk 1
• Urinalysis, including standard urine studies and microscopic analysis, offers valuable diagnostic information about renal and systemic conditions 1

Specialized Testing Based on Clinical Presentation

Cardiac Evaluation

• Cardiac markers, including high-sensitivity troponin assays and brain natriuretic peptide (BNP) or N-terminal proBNP, are vital for diagnosing cardiac conditions 1
• Blood gas analysis (arterial or venous) helps assess oxygenation and ventilation status 1

Neurological Evaluation

• In patients with seizures, laboratory tests should be ordered based on individual clinical circumstances, including assessment for metabolic abnormalities such as hypoglycemia, hyponatremia, hypocalcemia, and other electrolyte disturbances 2
• For patients with cognitive impairment, a cognitive lab panel should include CBC with differential, complete metabolic panel with electrolytes, glucose, calcium, magnesium, and phosphate, thyroid-stimulating hormone (TSH), vitamin B12 level, homocysteine level, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR) 2

Hematologic/Oncologic Evaluation

• For suspected hematologic malignancies, laboratory studies should include CBC with platelets and differential, blood chemistry profile, liver function tests, and disseminated intravascular coagulation panel (D-dimer, fibrinogen, PT, and PTT) 2
• Serum lactate dehydrogenase, uric acid, potassium, phosphates, and calcium should be measured to assess for tumor lysis syndrome 2

Pediatric Considerations

• "Micro" samples capability (samples <1 mL) is essential in pediatric care 1
• Age-appropriate reference ranges should be used for interpretation of results 1
• In cases of suspected child physical abuse, laboratory tests should include serum calcium, phosphorus, alkaline phosphatase, and potentially parathyroid hormone and 25-hydroxy-vitamin D to explore bone metabolism 2

Important Considerations for Laboratory Testing

• Laboratory tests should only be ordered if results will have clinical consequences and improve patient assessment 3
• The most frequent cause of an abnormal test result is physiologic variation, laboratory variation, or both, rather than disease 4
• Diagnosis is often established through history-taking (in up to 66% of cases) before laboratory testing, with physical examination and laboratory investigations each contributing to diagnosis in smaller percentages of cases 5
• Pre-analytical factors, such as hemolysis affecting potassium levels, must be considered to avoid inaccurate diagnoses 1
• Integration of laboratory test results with physical assessment findings is essential for effective patient care 6

Algorithm for Laboratory Test Selection

1. Begin with focused history and physical examination to generate specific clinical questions
2. Order core laboratory tests based on organ systems involved
3. Add specialized tests based on specific clinical suspicions
4. Interpret results in the context of the patient's clinical presentation
5. Consider follow-up testing only when initial results suggest a specific diagnosis requiring confirmation

Common Pitfalls to Avoid

• Ordering routine "panels" without specific clinical questions leads to unnecessary costs and potential false positives 1
• Failing to consider pre-analytical factors that may affect test results can lead to misdiagnosis 1
• Over-reliance on laboratory tests without proper clinical correlation may lead to diagnostic errors 3, 7
• Neglecting to establish appropriate follow-up for abnormal results can delay diagnosis and treatment 7
• Interpreting laboratory values in isolation without considering the patient's overall clinical picture can lead to inappropriate interventions 4, 7