What initial laboratory tests and medications are recommended for a new patient with unknown medical history?

Initial Laboratory Testing for New Patients with Unknown Medical History

For a new patient with completely unknown medical history, obtain a complete blood count (CBC), comprehensive metabolic panel (CMP), and urinalysis as your foundational laboratory assessment.

Core Laboratory Panel

Complete Blood Count (CBC)

• Order a CBC with differential as your first-line test to evaluate red blood cells, white blood cells, and platelets, which provides critical information about oxygen-carrying capacity, immune function, infection, anemia, and hematologic disorders 1.
• The CBC is one of the most powerful and frequently ordered diagnostic tools in medicine, capable of identifying or ruling out numerous diseases when combined with clinical assessment 2, 3, 4.
• Specifically evaluate hemoglobin, hematocrit, red blood cell indices (MCV, MCH, MCHC), white blood cell count with differential, and platelet count 5, 4.

Comprehensive Metabolic Panel (CMP)

• Obtain a CMP to assess kidney function, liver function, electrolytes, glucose, calcium, and albumin 1.
• This panel includes serum creatinine, blood urea nitrogen, electrolytes (sodium, potassium, chloride, bicarbonate), liver function tests (AST, ALT, alkaline phosphatase, bilirubin), albumin, and calcium 1.
• The CMP is essential for identifying metabolic abnormalities, organ dysfunction, and guiding medication safety decisions 1, 6.

Urinalysis with Calculated Creatinine Clearance

• Perform baseline urinalysis and calculate creatinine clearance or estimated glomerular filtration rate (eGFR) to assess kidney function, particularly important before initiating any nephrotoxic medications 1.
• This is especially critical in patients with risk factors for kidney disease or those who may require medications with renal toxicity potential 1.

Additional Screening Tests Based on Clinical Context

Infectious Disease Screening

• Consider tuberculosis screening with either tuberculin skin test (TST) or interferon-gamma release assay (IGRA) if the patient has unknown TB exposure history or risk factors 1.
• HIV serologic testing should be performed if no prior documentation exists, as this fundamentally changes management approach 1.

Endocrine Screening

• Order thyroid-stimulating hormone (TSH) if there are any signs or symptoms suggesting thyroid dysfunction (fatigue, weight changes, temperature intolerance, cardiac symptoms) 7.
• Obtain fasting blood glucose or hemoglobin A1c to screen for diabetes mellitus, particularly if the patient has cardiovascular risk factors, obesity, or family history 8.

Coagulation Studies

• Obtain a coagulation profile (PT/INR, PTT) if the patient requires any invasive procedures, has bleeding history, or takes anticoagulant medications 1.

Critical Pitfalls to Avoid

Over-Testing in Stable Patients

• Avoid ordering extensive laboratory panels in patients who appear clinically well without specific indications, as this wastes resources and may lead to unnecessary follow-up of clinically insignificant abnormalities 8.
• Do not order tests reflexively without clinical reasoning—each test should have a specific purpose tied to the patient's presentation 8.

Under-Testing in High-Risk Presentations

• Do not skip baseline testing in patients with concerning symptoms or risk factors, as missing systemic disease, infection, or organ dysfunction can delay critical diagnoses 8.
• Always obtain baseline renal and hepatic function before prescribing medications metabolized by these organs 1.

Missing Diabetes Screening

• Failure to check glucose or HbA1c can miss undiagnosed diabetes, which significantly impacts treatment decisions and prognosis across multiple conditions 8.
• This is particularly important in patients with infections, cardiovascular disease, or metabolic syndrome features 8.

Medication-Specific Considerations

Before Initiating Specific Therapies

• If considering immunosuppressive therapy, obtain baseline CBC including lymphocyte count, liver transaminases, and bilirubin levels 1.
• For nephrotoxic medications (such as tenofovir), ensure baseline urinalysis and calculated creatinine clearance are documented 1.
• Before abacavir therapy specifically, HLA-B*5701 testing must be performed to prevent hypersensitivity reactions 1.

Algorithmic Approach

Step 1: Obtain CBC, CMP, and urinalysis on every new patient with unknown history 1.

Step 2: Add TSH if any symptoms of thyroid dysfunction exist (cardiac symptoms, weight changes, fatigue) 7.

Step 3: Add fasting glucose or HbA1c if diabetes risk factors present (obesity, family history, cardiovascular disease) 8.

Step 4: Add infectious disease screening (HIV, TB) based on risk assessment and lack of prior documentation 1.

Step 5: Add coagulation studies if procedures planned or bleeding history exists 1.

Step 6: Add cardiac biomarkers (troponin) only if chest pain or cardiac ischemia suspected 7.

This structured approach ensures comprehensive baseline assessment while avoiding unnecessary testing, allowing you to identify occult disease, establish medication safety, and create an appropriate treatment plan.