Cognitive schemes and strategies in diagnostic and therapeutic decision making: a primer for trainees

Step	Clinical action	Scheme/cognitive aid
1	Gather information (history and physical)	–
2	Propose a diagnosis	Pattern-recognition hypothetico-deductive strategies and smart heuristics, rule-out worst scenario, red flags, etc.
3	Differential diagnosis	Differential diagnosis cognitive aids: anatomical, physiological, pathological
4	Order tests (rationally)	Frugal heuristics probability assessment: test sensitivity, specificity and likelihood ratios
5	Confirm and comprehensively give a diagnostic label	Guideline-friendly bedside diagnosis, aetiology, severity (BESD)
6	Therapeutic interventions	Contextual, patient-centred therapeutic cognitive aid: site of care, symptomatic, supportive, specific and speciality referral (5S)
7	Prepare for discharge	Assess response to treatment (subjective and objective), criteria for discharge, timing of follow-up (ACT)

Box 1

Summarizing the history and physical examination

Comprehensive but concise, text-book-like:

Must contain patient’s name, gender, age, ±occupation, ±nationality, ±racial/geographic origin, relevant past history/social history/family history, drug/allergic history, symptoms +duration—in technical terms, relevant physical signs in technical conclusive terms

Table 2

Differential diagnosis cognitive aids

Anatomical differential diagnosis	Physiological differential diagnosis	Aetiopathological differential diagnosis
Pain syndromes e.g. central chest pain may be categorized as arising from the heart, aorta, oesophagus, chest wall etc.	Shock this may be hypovolaemic, distributive, obstructive or cardiogenic	Congenital or hereditary
Swellings e.g. a neck swelling differential diagnosis will include the thyroid, lymph nodes, vascular, skin etc.	Thrombosis this may be related to a vessel wall pathology, blood constituents or flow rate	Acquired
		1. Traumatic
		2. Infective: viral, bacterial etc.
		3. Inflammatory/auto-immune
		4. Vascular/degenerative
		5. Neoplastic/para-neoplastic
		6. Metabolic/endocrine
		7. Drug-induced/poisoning
		8. Deficiency diseases
		9. Psychogenic
		10. Idiopathic/cryptogenic

Table 3

Sensitivity, specificity and likelihood ratios: definitions and examples

Sensitivity	Example in a group of 100 patients with bacterial pneumonia, 80 had a raised C-reactive protein CRP: the sensitivity of CRP for diagnosing bacterial pneumonia is thus 80 %
How often is the test result correct for persons in whom the disease is known to be present?
Sensitivity—the proportion of people with disease who have a positive test
Specificity	Example in a group of 100 patients without pneumonia, 10 had a raised C-reactive protein CRP: the specificity of CRP for correctly excluding pneumonia is thus 90 %
How often is the test result correct for persons in whom the disease is known to be absent?
Specificity—the proportion of people without the disease who have a negative test
Likelihood ratio	Example A raised jugular venous pressure (JVP) in a patient with a history suggestive of congestive heart failure (CHF) has a positive likelihood ratio of 5.8 and a negative ratio of 0.66. Thus the presence of a raised JVP rules-in the diagnosis of CHF. Its absence is not as useful in ruling it out
The likelihood that a given test result would be expected in a patient with the target disorder compared with the likelihood that the same result would be expected in a patient without that disorder.
In general, a positive likelihood ratio of 4 or more is useful in ruling-in the target disorder. A negative likelihood ratio of <0.3 is useful in ruling-out the target disorder

Table 4

A case scenario illustrating the use of the ‘technical’ expert summary, BESD, pathological differential diagnosis and 5S therapeutic interventions

• 67-year-old male
• Bird/pigeon breeder, smoker
• 3-day history of fever, cough with yellow sputum, left stabbing chest pain that is worse with breathing and coughing and breathlessness
• Clinically, breathless, cyanosed, disoriented to time, person and place,
Temperature 39.1 °C
• BP 86/50 mmHg, RR 32/min, bilateral coarse crepitations, bronchial breathing left lower zone
• Chest X-ray: left basal consolidation
Summary
67-year-old, smoker and bird-breeder presenting with a 3-day history of productive cough, dyspnoea and left pleuritic chest pains
Clinically confused, cyanosed, febrile, tachypnoiec and hypotensive with signs of left lower zone consolidation
1. Bedside-clinical diagnosis	Community acquired pneumonia with septic shock
2. Cause/precipitant	Chlamydia psittaci
	Aetio-pathological differential diagnosis
	Other Infections: e.g. avian flu, cryptococcal infection
	Inflammatory e.g. collagenosis, allergic alveolitis
	Vascular e.g. pulmonary embolism
	Neoplastic, drug-induced etc.
3. Severity	Life-threatening (CURB-65 = 4)
4. Site of care	ICU
5. Symptomatic	Analgesia, anti-pyretic
6. Supportive	Oxygen, intravenous fluids
7. Specific	Antibiotics
8. Speciality referral	Intensive therapy unit, pulmonary service