Medical Decision Making

On this page

Diagnostic Tests - Numbers Game

  • Basis: 2x2 table (Disease vs. Test: TP, FP, FN, TN).
  • Sensitivity (Sn): $TP / (TP+FN)$. Detects disease.
    • 📌 SNOUT: Sensitive test, Negative result, rules OUT.
  • Specificity (Sp): $TN / (TN+FP)$. Confirms absence.
    • 📌 SPIN: Specific test, Positive result, rules IN.
  • PPV (Positive Predictive Value): $TP / (TP+FP)$. Prob. disease if test +ve. Varies with prevalence.
  • NPV (Negative Predictive Value): $TN / (TN+FN)$. Prob. no disease if test -ve. Varies with prevalence.
  • Likelihood Ratios (LR): Link pre- to post-test probability.
    • LR+: $Sn / (1-Sp)$. >10 strong for disease.
    • LR-: $(1-Sn) / Sp$. <0.1 strong against disease. 2x2 Contingency Table for Diagnostic Test Evaluation

⭐ Prevalence ↑ → PPV ↑, NPV ↓.

Decision Thresholds - Choice Points

  • Critical probability points guiding decisions: withhold action, test, or treat.
  • Test Threshold ($T_{test}$):
    • If P(Disease) < $T_{test}$: No further testing or treatment. Disease considered ruled out.
  • Treatment Threshold ($T_{treat}$):
    • If P(Disease) > $T_{treat}$: Initiate treatment empirically. Disease considered ruled in.
  • Test-Treat Zone:
    • If $T_{test}$ ≤ P(Disease) ≤ $T_{treat}$: Optimal to perform diagnostic testing.
  • Influencing factors:
    • Disease: Prevalence, severity.
    • Test: Accuracy (Sn, Sp), risks, cost.
    • Treatment: Efficacy, risks, benefits, cost.

⭐ The treatment threshold ($T_{treat}$) is lowered by ↑disease severity, ↑treatment efficacy, or ↓treatment risk.

Decision thresholds: test, uncertain, and treatment

Clinical Scores & EBM - Evidence Edge

  • Clinical Scores: Objectify risk/diagnosis (e.g., Wells' for DVT/PE, CURB-65 for pneumonia severity).
    • Metrics: Sensitivity $TP/(TP+FN)$, Specificity $TN/(TN+FP)$.
    • PPV $TP/(TP+FP)$, NPV $TN/(TN+FN)$.
    • Likelihood Ratios: $LR+ = Sens/(1-Spec)$; $LR- = (1-Sens)/Spec$.
  • EBM: Integrates best evidence, clinical expertise, patient values.
    • PICO: Patient, Intervention, Comparison, Outcome.
    • Hierarchy (Top to Bottom): Meta-analysis/Systematic Review → RCT → Cohort → Case-Control → Case Series → Expert Opinion.
    • Key EBM stat: Number Needed to Treat (NNT) = $1/ARR$.
    • 📌 FINER criteria (Research Q): Feasible, Interesting, Novel, Ethical, Relevant. Hierarchy of Evidence in Evidence-Based Medicine

⭐ Odds Ratio (OR) is used in case-control studies; Relative Risk (RR) in cohort studies. For rare diseases ($prevalence < 10%$), $OR \approx RR$

Cognitive Biases & SDM - Mind Matters

  • Cognitive Biases: Systematic thinking errors impairing clinical judgment.
    • Anchoring: Over-relying on initial info.
    • Availability: Overestimating based on easily recalled examples.
    • Confirmation: Seeking info confirming existing beliefs.
    • Premature Closure: Accepting diagnosis early, missing alternatives.
    • Framing Effect: Decisions influenced by info presentation.
    • Mitigation: Metacognition, checklists, second opinions.
  • Shared Decision Making (SDM): Clinicians and patients collaboratively make healthcare decisions.
    • Involves discussing options, evidence, benefits/risks.
    • Aligns care with patient values and preferences.
    • Improves satisfaction and adherence. 📌 Remember "BRAN": Benefits, Risks, Alternatives, Nothing.

⭐ SDM is not just an option but an ethical imperative in patient-centered care.

High‑Yield Points - ⚡ Biggest Takeaways

  • Bayes' Theorem is crucial for post-test probability calculation using likelihood ratios.
  • Understand Sensitivity (Sn), Specificity (Sp), PPV, and NPV for test interpretation.
  • ROC curves help compare diagnostic tests; Area Under Curve (AUC) quantifies overall accuracy.
  • Decision analysis often employs decision trees to model choices and potential outcomes.
  • The threshold model (test/treat thresholds) guides diagnostic and therapeutic actions.
  • Recognize common cognitive biases (e.g., anchoring, availability) that can affect clinical judgment.
  • Apply Evidence-Based Medicine (EBM) principles, respecting the hierarchy of evidence for decisions.
Rezzy AI Tutor

Have doubts about this lesson?

Ask Rezzy, our AI tutor, to explain anything you didn't understand

Practice Questions: Medical Decision Making

Test your understanding with these related questions

Specificity of a diagnostic test is defined as:

1 of 5

Flashcards: Medical Decision Making

1/5

Pellagra (from severe B3 deficiency) may be caused by _____ disease, malignant carcinoid syndrome, and isoniazid therapy

TAP TO REVEAL ANSWER

Pellagra (from severe B3 deficiency) may be caused by _____ disease, malignant carcinoid syndrome, and isoniazid therapy

Hartnup

browseSpaceflip

Enjoying this lesson?

Get full access to all lessons, practice questions, and more.

Start For Free