Medical Investigations: A Systematic Approach
Medical investigations are diagnostic procedures—including laboratory tests, imaging studies, and specialized examinations—used to detect, diagnose, characterize, or monitor specific medical conditions, with the goal of reducing diagnostic uncertainty and guiding clinical management. 1
Core Components of Medical Investigations
Laboratory Testing
Laboratory investigations form the foundation of diagnostic workup and include blood tests, body fluid analysis, and microbiological studies. 2
Complete Blood Count (CBC) assesses hemoglobin, hematocrit, red blood cell count, mean corpuscular volume (MCV), white blood cell count with differential, and platelet count to evaluate for anemia, infection, hematologic abnormalities, and clotting disorders 3, 4
Comprehensive Metabolic Panel evaluates electrolytes (sodium, potassium, chloride, bicarbonate), glucose, renal function (creatinine, blood urea nitrogen), and liver function tests (ALT, AST, alkaline phosphatase, bilirubin, albumin) 3
Inflammatory markers such as C-reactive protein (CRP) assess for inflammatory or infectious processes 5, 3
Blood gas analysis measures pH, base excess, and lactate levels to evaluate acid-base status and tissue perfusion 3
Microbiological Investigations
Microbiological testing identifies infectious pathogens through culture, PCR, and serological methods. 5
CSF analysis via lumbar puncture should include cell count with differential, protein, glucose, Gram stain, bacterial culture, and PCR for HSV-1, HSV-2, VZV, and enteroviruses as first-line testing 5, 6
Stool studies include culture for bacterial pathogens and testing for Clostridioides difficile toxin 5
Respiratory specimens (throat swabs, nasopharyngeal aspirates) undergo PCR or culture for viral pathogens including influenza, adenovirus, and respiratory syncytial virus 5
Imaging Studies
Imaging investigations visualize anatomical structures and pathological processes to complement laboratory findings. 5, 3
Plain radiographs (chest X-ray, abdominal X-ray) assess for radiopaque foreign bodies, obstruction, perforation, or toxic megacolon 5, 3
Computed tomography (CT) is indicated when complications such as perforation, abscess, or other serious conditions are suspected 3
Magnetic resonance imaging (MRI) with conventional sequences (T1/T2, FLAIR), diffusion-weighted imaging, and gadolinium enhancement evaluates neurological conditions 6
Specialized Diagnostic Procedures
Specialized investigations target specific organ systems or disease processes. 5, 6
Electroencephalography (EEG) diagnoses seizure disorders or evaluates acute confusional states, with abnormalities present in 60-70% of patients with seizure disorders 6
Flexible sigmoidoscopy with biopsies obtains tissue for urgent histology and specific pathogen testing (e.g., CMV) in inflammatory bowel disease 5
Tuberculin skin testing or interferon-gamma release assays screen for latent tuberculosis infection in contacts of infectious cases 5
Clinical Application Framework
Baseline Investigation Protocol
For acute presentations requiring urgent treatment, obtain baseline investigations without delaying therapy. 5
The British Society of Gastroenterology recommends the following baseline workup for acute severe colitis 5:
- Full blood count, CRP, urea and electrolytes, liver function tests
- Stool culture and C. difficile testing
- Abdominal radiography or CT
- Screening for hepatitis B and C, HIV, varicella-zoster virus
- Tuberculosis screening with chest X-ray and interferon-gamma release assay
Risk-Stratified Investigation Approach
Tailor the extent of investigations to clinical severity and risk factors. 3
For uncomplicated presentations: CBC, electrolytes, and renal function tests suffice 3
For moderate to severe symptoms or concerning features: Add complete metabolic panel including liver function tests, CRP, and blood gas analysis 3
For suspected complications: Proceed directly to advanced imaging (CT) rather than sequential testing 3
Diagnostic Accuracy Considerations
The diagnostic yield of investigations varies significantly by clinical context, with medical history providing the diagnosis in 76% of cases, physical examination in 12%, and laboratory investigation in 11%. 7
Laboratory and imaging investigations primarily serve to exclude alternative diagnoses, increase diagnostic confidence, and guide treatment rather than establish the primary diagnosis 7
Sensitivity and specificity of diagnostic tests must be interpreted in the context of disease prevalence, as false positive rates increase when prevalence is low (screening scenarios) and false negative rates increase when prevalence is high 8
Critical Pitfalls to Avoid
Never delay urgent treatment while awaiting investigation results in time-sensitive conditions. 5
Do not withhold corticosteroids in acute severe colitis while awaiting stool culture results 5
Contrast studies should not delay other investigations in acute presentations 3
Always exclude alternative diagnoses before attributing symptoms to the suspected condition. 6
In suspected lupus cerebritis, rule out CNS infections, metabolic disturbances, steroid-induced psychosis, and thrombotic events before confirming the diagnosis 6
Persistent laboratory abnormalities despite symptom resolution mandate investigation for alternative etiologies 3
Recognize that normal investigation results do not exclude disease. 6