Laboratory Tests for Confirming a Diagnosis
The appropriate laboratory tests to confirm a diagnosis depend on the specific condition being investigated, but should always include a complete blood count (CBC), comprehensive metabolic panel, and targeted tests based on the suspected condition.
Core Laboratory Tests for Initial Diagnostic Workup
Basic Tests for All Patients
Complete Blood Count (CBC) with differential 1
- Provides information about red blood cells, white blood cells, and platelets
- Helps identify infections, anemias, leukemias, and other hematologic disorders
- Includes examination of peripheral blood smear for abnormalities
Comprehensive Metabolic Panel 1
- Electrolytes (sodium, potassium, chloride, bicarbonate)
- Kidney function (BUN, creatinine)
- Liver function tests (ALT, AST, alkaline phosphatase, bilirubin)
- Glucose, calcium, albumin
- Uric acid and lactate dehydrogenase (LDH)
Urinalysis 1
- Complete urinalysis including microscopic examination
- 24-hour urine collection for protein quantification if indicated
Condition-Specific Laboratory Tests
For Suspected Cardiac Conditions 1
- Cardiac biomarkers (troponin, NT-proBNP)
- Lipid panel
- Natriuretic peptide levels (BNP or NT-proBNP)
- Useful for diagnosing heart failure, especially in urgent care settings
- Note: BNP levels may be less elevated in heart failure with preserved ejection fraction
For Suspected Neurological Conditions 1
- Cerebrospinal fluid (CSF) examination
- Protein levels (albumino-cytological dissociation is classic in Guillain-Barré syndrome)
- Cell count (normal or mild pleocytosis in GBS)
- Anti-ganglioside antibodies (particularly anti-GQ1b for Miller Fisher syndrome)
For Suspected Hematologic Disorders 1
- Serum protein electrophoresis (SPEP)
- Immunofixation electrophoresis (IFE)
- Serum free light chain assay
- Beta-2 microglobulin
- Bone marrow aspirate and biopsy with cytogenetics when indicated
For Suspected Eosinophilic Disorders 1
- Serum tryptase levels
- Vitamin B12 levels
- IgE levels
- Specialized testing:
- Fluorescence in situ hybridization (FISH)
- Reverse transcription PCR (RT-PCR) for tyrosine kinase fusion gene rearrangements
For Suspected Infectious Processes 1
- Culture and sensitivity of relevant specimens
- Pus sampling in high-risk patients with abscesses
- Serologic testing for specific pathogens based on clinical presentation
Special Considerations
Timing of Laboratory Tests
- Some tests may yield false negatives early in disease course
- CSF protein may be normal in 30-50% of Guillain-Barré patients in the first week 1
- Serial testing may be necessary to confirm diagnosis
Avoiding Unnecessary Testing
- Approximately 45% of routine admission tests are ordered without clear medical indications 2
- Only 0.5% of routine tests without specific indications lead to changes in treatment
- Target testing based on clinical suspicion rather than ordering panels indiscriminately
Interpretation Pitfalls
- Natriuretic peptide levels may be affected by age, sex, weight, and renal function 1
- Elevated natriuretic peptides should not be used in isolation to confirm or exclude heart failure
- Laboratory results must always be interpreted in clinical context
Follow-Up Testing
After initial diagnostic tests, follow-up testing should be guided by initial results and clinical response. Monitor disease progression and treatment response with appropriate serial testing of the most relevant parameters.
Remember that laboratory testing is just one component of diagnosis and should be integrated with clinical findings, imaging studies, and other diagnostic modalities as appropriate for the suspected condition.