Quality-adjusted life year Study Guide

📖 Core Concepts Quality‑Adjusted Life Year (QALY) – a single metric that merges quantity of life (years lived) with quality of those years (utility). One QALY = one year in perfect health. Utility weight – a numeric value (0 = death, 1 = perfect health) reflecting how people value a specific health state; can be negative for states “worse than death.” Cost‑Utility Analysis (CUA) – economic evaluation that expresses outcomes in QALYs and relates them to costs. Incremental Cost‑Effectiveness Ratio (ICER) – the extra cost per extra QALY gained when switching from one intervention to another. Decision‑making threshold – a monetary ceiling (e.g., £20,000‑£30,000 per QALY in the UK) that a health agency uses to deem an intervention “worth it.” --- 📌 Must Remember QALY formula: $$\text{QALY}= \text{Utility} \times \text{Years}$$ Utility 1 = perfect health, 0 = death, <0 = worse than death. ICER = (Cost\A – Cost\B) / (QALY\A – QALY\B). Threshold rule: if ICER ≤ threshold → intervention is typically accepted. Main utility‑elicitation methods: Time‑Trade‑Off (TTO), Standard Gamble (SG), Visual Analogue Scale (VAS), Descriptive systems (e.g., EQ‑5D). Critics: QALYs may under‑represent mental health, penalize disabilities, and favor younger/healthier populations. --- 🔄 Key Processes Eliciting Utility Weights Choose a method (TTO, SG, VAS, EQ‑5D). Survey patients/general public. Convert responses to a utility score (0‑1, possibly negative). Calculating QALYs for an Intervention Identify health states over time. Multiply each state’s utility by the duration spent in that state. Sum across all periods: $$\text{Total QALY}= \sum{i}\text{Utility}i \times \text{Years}i$$ Conducting a Cost‑Utility Analysis Gather total costs (direct, indirect). Compute total QALYs for each comparator. Derive ICER. Compare ICER to the policy threshold. --- 🔍 Key Comparisons Time‑Trade‑Off vs. Standard Gamble TTO: trade years of life for perfect health; assumes risk neutrality. SG: choose between certain ill health and a probabilistic cure/death; incorporates risk attitudes. QALY vs. DALY QALY = years adjusted for quality (higher is better). DALY = years lost due to disability + years lost to premature death (higher is worse). EQ‑5D vs. VAS EQ‑5D: structured descriptive system (5 dimensions) → more comparable across studies. VAS: simple 0‑100 rating → more subjective, less standardized. --- ⚠️ Common Misunderstandings “A QALY is a dollar value.” – QALY is a health outcome, not money; it is paired with cost in CUA. “Utility = quality of life.” – Utility captures preference for a health state, not all aspects of wellbeing (e.g., social support). “Negative utilities mean the person is dead.” – Negative values indicate states worse than death, not death itself. “Higher ICER always means a bad intervention.” – An ICER may be acceptable if it is below the relevant threshold. --- 🧠 Mental Models / Intuition “Life‑year buckets” – Imagine each year of life as a bucket that can be filled (utility = 1) or partially filled (utility < 1). QALYs count the total volume of water across all buckets. “Cost per filled bucket” – ICER is simply the extra dollars you spend to fill an additional portion of a bucket (extra QALY). --- 🚩 Exceptions & Edge Cases Negative utilities – Some rare health states are valued below 0; include them directly in the QALY sum (they subtract from total QALYs). Multiple health states – When a patient moves between states, add each period’s utility×years; do not average utilities before multiplying. Equity weighting – Some analyses apply extra weight to QALYs for disadvantaged groups (not captured in the basic formula). --- 📍 When to Use Which Choose TTO when you need a time‑based trade‑off and the population is risk‑neutral. Choose SG when capturing risk attitudes is crucial (e.g., life‑saving treatments). Use VAS for rapid, low‑cost screening of many health states. Use EQ‑5D for large‑scale health‑technology assessments requiring comparability across studies. --- 👀 Patterns to Recognize Pattern: “Utility × Years = QALYs” – any question giving a utility and a duration can be solved instantly. Pattern: “ICER ≤ Threshold → Accept” – look for the numeric threshold; if the calculated ICER is lower, the intervention passes. Pattern: “Negative utility → subtract from total QALYs” – watch for “worse than death” states in multi‑period problems. --- 🗂️ Exam Traps Trap 1: Swapping numerator/denominator in ICER (cost difference ÷ QALY difference, not the reverse). Trap 2: Treating a VAS score (0‑100) as a utility without dividing by 100. Trap 3: Forgetting to sum QALYs across periods; using an average utility instead. Trap 4: Assuming a higher ICER is automatically “bad” without checking the policy threshold. Trap 5: Misreading a negative utility as zero; it should reduce total QALYs. ---