What are the steps to diagnose a medical condition?

How to Diagnose a Medical Condition

The medical history is the single most important diagnostic tool, leading to the correct diagnosis in approximately 76-79% of cases, and should be prioritized over physical examination and laboratory testing. 1, 2

Step 1: Obtain a Comprehensive Medical History

The history alone establishes the diagnosis in three-quarters of patients and must be performed systematically 1, 2:

Essential History Components

Presenting Complaint Details:

• Timing: Onset (acute vs. subacute vs. chronic), duration, and periodicity of symptoms 3
• Location and radiation: Document if symptoms follow anatomical distributions (e.g., nerve territories, vascular territories) 3
• Quality and severity: Characterize the nature of symptoms using patient's own descriptors 3
• Aggravating and relieving factors: Identify specific triggers including positional changes, physical activity, food intake, temperature, stress, and time of day 3

Associated Symptoms:

• Document all accompanying symptoms in affected and adjacent systems 3
• Screen for systemic symptoms (fever, weight loss, fatigue) 3
• Assess impact on sleep, mood, concentration, and quality of life 3

Past Medical and Surgical History:

• Previous similar episodes or related conditions 3
• Chronic diseases that may contribute to current presentation 3

Medication History:

• Current medications, dosages, and adherence 3
• Recent medication changes 3
• Over-the-counter and complementary therapies 3

Family and Social History:

• Genetic predisposition to relevant conditions 3
• Occupational and environmental exposures 3
• Recent infections (within 6 weeks for conditions like Guillain-Barré syndrome) 4

Step 2: Perform Targeted Physical Examination

Physical examination leads to diagnosis in 8-12% of cases and increases diagnostic confidence substantially 1, 2:

Systematic Examination Approach

Inspection:

• Visual assessment for color changes, swelling, deformities, skin lesions, and asymmetry 3
• Document changes over time (e.g., aneurysm size progression) 3

Palpation:

• Assess temperature, texture, tenderness, masses, and pulsations 3
• Evaluate for trigger points and areas of referred pain 3
• Check muscle strength using standardized grading scales 4

Auscultation:

• Listen for abnormal sounds (bruits, murmurs, bowel sounds) that indicate pathology 3
• Distinguish normal from pathological findings (e.g., low-frequency vs. high-frequency bruits) 3

Neurological Examination (when indicated):

• Test cranial nerves systematically, particularly facial nerve function 3, 4
• Assess reflexes, with particular attention to diminished or absent reflexes in suspected neuropathies 4
• Evaluate sensory function and distribution of deficits 4

Functional Assessment:

• Observe gait, range of motion, and ability to perform specific tasks 4
• Assess respiratory function including vital capacity and breath counting in suspected neuromuscular conditions 4

Step 3: Order Appropriate Diagnostic Testing

Laboratory and imaging investigations lead to diagnosis in 11-13% of cases but are critical for confirming diagnoses and excluding alternatives 1, 2:

First-Tier Testing

Basic Laboratory Studies:

• Complete blood count to detect cytopenias or systemic inflammation 4, 5
• Basic metabolic panel including electrolytes, glucose, kidney and liver function 4, 5
• Thyroid function tests when metabolic disorders are suspected 5

Condition-Specific Testing:

• Creatine kinase for suspected muscle involvement 4
• Cerebrospinal fluid analysis when central nervous system pathology is suspected (looking for albumino-cytological dissociation in Guillain-Barré syndrome) 4
• Skin prick testing or specific IgE for suspected food allergies 3

Advanced Diagnostic Studies

Electrodiagnostic Studies:

• Nerve conduction studies and electromyography for suspected neuropathies 4
• Electrocardiography for cardiac symptoms or autonomic dysfunction 4

Imaging Studies:

• Plain radiographs for dental and bony pathology 3
• Ultrasound for soft tissue and salivary gland evaluation 3
• CT or MRI when structural abnormalities are suspected 3
• Echocardiography to assess cardiac structure and function 3

Specialized Testing:

• Component-resolved diagnostics for food allergies when initial testing is equivocal 3
• Oral food challenges as the gold standard for food allergy diagnosis 3
• Whole exome sequencing for suspected mitochondrial disorders with complex presentations 5

Step 4: Integrate Findings and Establish Diagnosis

Synthesize all data systematically:

• The history provides the diagnostic framework in 76-79% of cases 1, 2
• Physical examination increases diagnostic confidence from 7.1/10 to 8.2/10 2
• Investigations further increase confidence to 9.3/10 and exclude alternative diagnoses 2

Critical Pitfalls to Avoid

Do not order investigations before obtaining a thorough history and examination - this leads to unnecessary testing, increased costs, and potential patient harm from false positives 6, 7

Do not dismiss diagnoses based on normal early testing - for example, CSF protein may be normal in the first week of Guillain-Barré syndrome 4

Do not rely solely on physical findings in isolation - abnormal surveillance tests should be corroborated by clinical findings before proceeding to invasive testing 3

Always consider life-threatening conditions first - assess respiratory function and autonomic stability immediately in suspected Guillain-Barré syndrome, as these determine mortality risk 4

Document findings systematically - maintain detailed records of physical examination findings over time to track progression or resolution 3

Reconsider the diagnosis when atypical features are present - marked asymmetry, unusual timing, or unexpected test results should prompt reassessment 4