What laboratory tests should be routinely checked in a patient presenting to the emergency unit, regardless of age or medical history?

Routine Laboratory Testing in the Emergency Department

Routine laboratory testing should NOT be performed universally on all emergency department patients; instead, laboratory testing must be guided by the patient's clinical presentation, history, and physical examination findings. 1

Core Principle: Selective Testing Based on Clinical Assessment

The evidence consistently demonstrates that indiscriminate routine laboratory testing in the ED is of very low yield and generates more false positives than clinically significant findings. 1

Key Evidence Against Routine Testing:

• False positive rate exceeds true positives by 8:1 when routine testing is performed without clinical indication 1
• History and physical examination predict the majority of laboratory abnormalities with 94% sensitivity 1, 2
• Most abnormal results obtained from routine screening are clinically insignificant 1

Essential Laboratory Tests: The Minimal Core Panel

Based on survey data of emergency physicians, only a limited number of tests are truly indispensable for ED decision-making: 3

Primary Essential Tests:

• Complete blood count (CBC) - specifically hemoglobin and leukocytes 1, 3
• Basic metabolic panel - sodium, potassium, urea/creatinine 1, 3
• C-reactive protein (CRP) 3
• Glucose (often via point-of-care testing) 1, 3

Secondary Essential Tests (Based on Clinical Context):

• Troponin - for suspected acute coronary syndrome 3
• Lactate and blood gas analysis - for patients with signs of sepsis or systemic infection 1
• Liver function tests - when hepatic pathology suspected 1

Clinical Scenarios Requiring Laboratory Testing

1. Patients with Psychiatric Complaints

Do NOT perform routine laboratory testing in alert, cooperative patients with normal vital signs and noncontributory history/physical examination. 1

Exception: Consider selective testing for:

• Thyroid function in patients with affective disorders 1
• Urinalysis in women with psychiatric complaints 1
• Glucose in elderly patients with new-onset psychosis (to exclude undetected diabetes) 2

2. Patients with New-Onset Seizures

Minimal testing required if patient has returned to baseline neurologic status and has normal vital signs. 1

Recommended selective tests:

• Glucose - most frequent unexpected abnormality (though rare: 1-2 cases per 100-250 patients) 1
• Sodium - hyponatremia occasionally found but usually predicted by history 1
• Pregnancy test in women of childbearing age (affects treatment decisions) 1

NOT recommended routinely:

• Calcium, magnesium, phosphate - no prospective evidence supports routine testing 1
• Urine drug screen - no demonstrated benefit for routine use 1

3. Patients Requiring Hospital Admission

More comprehensive testing is appropriate for admitted patients: 1

• Complete blood count with differential 1
• Comprehensive metabolic panel (glucose, sodium, liver and renal function, electrolytes) 1
• Oxygen saturation assessment 1
• Arterial blood gas in severe illness or chronic lung disease 1

4. Patients with Suspected Severe Infection/Sepsis

Expanded laboratory evaluation indicated: 1

• Complete blood count 1
• Serum creatinine and electrolytes 1
• Inflammatory markers (CRP, procalcitonin) 1
• Blood gas analysis 1
• Serum glucose, hemoglobin A1c, urine ketones (to investigate undetected diabetes) 1
• Blood cultures (two sets before antibiotics) 1

Point-of-Care Testing (POCT) Strategy

POCT should be reserved for urgent obligatory parameters requiring results within 60 minutes for immediate therapeutic decisions. 4

Appropriate POCT Applications:

• Glucose 4, 3
• Cardiac biomarkers in suspected ACS 4, 5
• Basic metabolic parameters in critically ill patients 4
• Lactate in sepsis 5

POCT reduces time to clinical decision significantly for bacterial infections (median reduction from 7.5 to 4.6 hours), but evidence is mixed for other conditions. 6

Common Pitfalls to Avoid

Critical Errors:

1. Ordering comprehensive panels "just in case" - generates false positives requiring unnecessary follow-up 1
2. Assuming elderly patients with psychiatric symptoms have primary psychiatric disease - 20% have medical etiologies requiring targeted laboratory evaluation 2
3. Performing lumbar puncture routinely in seizure patients - only indicated if immunocompromised, febrile, or altered mental status 1
4. Delaying treatment while awaiting laboratory results - clinical assessment should drive initial management 1

Best Practice Approach:

• Begin with focused history and physical examination targeting specific red flags: altered mental status, fever, focal neurologic deficits, abnormal vital signs 1
• Order tests that will change management within the ED timeframe 4, 3
• Recognize that normal vital signs and nonfocal examination have high negative predictive value for serious pathology 1

Practical Algorithm for ED Laboratory Testing

1. Perform clinical assessment first - vital signs, mental status, focused examination 1
2. If patient is alert, cooperative, with normal vitals and nonfocal exam: Minimal or no laboratory testing 1
3. If any clinical red flags present: Targeted testing based on specific findings 1
4. If admission planned: Comprehensive panel appropriate for inpatient management 1
5. If severe illness/sepsis suspected: Expanded evaluation including inflammatory markers and cultures 1

The overarching principle: Let clinical judgment, not reflexive ordering, guide laboratory utilization in the emergency department. 1