USMLE Step 3 cheat sheet

Sensitivity = TP/(TP+FN); specificity = TN/(TN+FP); PPV = TP/(TP+FP) rises with prevalence while NPV falls.

Post-test odds = pretest odds × likelihood ratio; LR+ = sensitivity/(1 − specificity); LR− = (1 − sensitivity)/specificity.

ARR = control event rate − treatment event rate; NNT = 1/ARR; ARI = treatment event rate − control event rate; NNH = 1/ARI.

Case-control studies yield odds ratios; cohort studies yield relative risk and attributable risk; randomized trials yield RR, ARR, and NNT.

Alpha (conventionally 0.05) is the type I error rate; beta is the type II error rate; power = 1 − beta, conventionally 0.80.

Screening biases: lead-time bias (earlier diagnosis without longer survival), length-time bias (slow-growing disease overrepresented), selection bias (volunteers are healthier).

Trial phases: I = dosing and safety; II = efficacy and optimal dose; III = comparison with standard in a large RCT; IV = post-marketing surveillance.

Medicare covers age 65 or older, end-stage renal disease, and ALS; Medicaid is joint state-federal and income-based; EMTALA requires stabilization regardless of ability to pay.

Informed consent requires capacity, disclosure, comprehension, and voluntariness; the Tarasoff duty permits warning an identifiable intended victim of credible serious violence.

STEMI: chewed aspirin 162–325 mg plus a P2Y12 inhibitor and anticoagulation; primary PCI with door-to-balloon ≤90 min, or fibrinolysis within 30 min when PCI is not available within 120 min.

Acute ischemic stroke: IV thrombolysis within 4.5 h of last known well if BP <185/110 and no hemorrhage; door-to-needle ≤60 min; thrombectomy up to 24 h with large-vessel occlusion.

Sepsis: 30 mL/kg crystalloid for hypotension or lactate ≥4 mmol/L, broad-spectrum antibiotics within 1 h, target MAP ≥65 mmHg; add vasopressors when fluids are insufficient.

DKA: isotonic fluids, IV regular insulin 0.1 units/kg/h once potassium ≥3.3 mEq/L, add dextrose when glucose <200–250 mg/dL, and continue insulin until the anion gap closes.

Hyperkalemia with ECG changes: IV calcium gluconate first for membrane stabilization, then insulin/glucose and albuterol to shift potassium intracellularly; dialysis for refractory or renal failure.

Chronic hyponatremia: correct by no more than 8–10 mEq/L per 24 h; symptomatic hyponatremia gets 3% saline 100–150 mL boluses repeated until symptoms improve.

COPD exacerbation: titrate oxygen to SpO2 88–92%, give inhaled bronchodilators, systemic corticosteroids, antibiotics, and noninvasive ventilation for respiratory acidosis.

Atrial fibrillation: anticoagulate when CHA2DS2-VASc is ≥2 in men or ≥3 in women; rate control with a beta-blocker or nondihydropyridine calcium channel blocker.

Adrenal crisis: hydrocortisone 100 mg IV immediately with isotonic saline and dextrose; draw cortisol and ACTH but never delay steroids for the results.