QALY (Quality Adjusted Life Year)

A quality adjusted life year (QALY) is a measure of health outcome that combines both the quantity and quality of life into a single metric, used in health economics to evaluate and compare the value of medical interventions. One QALY represents one year of life lived in perfect health. A year lived in less than perfect health is worth less than one QALY, with the exact value determined by a utility weight that reflects the severity of the health state on a scale anchored at zero (death) and one (perfect health); some very severe states are valued below zero.

QALYs provide a common currency for comparing the health benefits of interventions across different diseases, conditions, and treatment modalities. Without a generic outcome measure like the QALY, comparing the value of a cancer treatment against a cardiovascular prevention program or a mental health intervention would be impossible on a consistent basis. Health technology assessment agencies, including NICE, Canada's Drug Agency (formerly CADTH), and PBAC, use QALYs as the primary outcome measure in cost effectiveness analyses that inform coverage, reimbursement, and pricing decisions. The QALY framework explicitly weights health states by stated preferences, usually those of the general public elicited through a country's value set, rather than by clinical judgment.

QALYs are calculated by multiplying the time spent in a health state by the utility weight associated with that state. If a patient lives for three years in a health state with a utility weight of 0.7, the patient accrues 2.1 QALYs (3 multiplied by 0.7). If an intervention improves the utility from 0.7 to 0.85 for those three years, the incremental QALY gain is 0.45 (3 multiplied by the difference of 0.15).

Utility weights are derived from preference based instruments administered to patients or the general public. The EQ 5D is the most widely used instrument, measuring five dimensions of health (mobility, self care, usual activities, pain or discomfort, and anxiety or depression) at several severity levels. Other instruments include the SF 6D (derived from the SF 36 health survey) and the HUI (Health Utilities Index). Utilities can also be elicited directly using choice based methods such as the standard gamble or time trade off; a visual analogue (rating) scale is also used, but because it involves no trade off, many economists do not treat its values as utilities.

In economic evaluations, QALYs are used in the denominator of the incremental cost effectiveness ratio (ICER), calculated as the difference in costs divided by the difference in QALYs between two interventions. The resulting ICER is compared against a willingness to pay threshold to determine whether the intervention represents good value. For example, if a new drug costs $30,000 more than the standard treatment and produces 0.5 additional QALYs, the ICER is $60,000 per QALY.

When health outcomes extend over many years, future QALYs are discounted to reflect time preference, at the annual rate set by the relevant guideline (for example, 3.5% for both costs and QALYs at NICE in England and 1.5% at Canada's Drug Agency, formerly CADTH). This means a QALY gained ten years in the future is worth less in present value terms than a QALY gained today. Discounting is standard practice in health economic evaluations and is specified in most national guidelines.

Use QALYs as the health outcome measure in cost effectiveness and cost utility analyses, particularly when the intervention affects both survival and health related quality of life. QALYs are the standard outcome for submissions to many health technology assessment bodies and for published economic evaluations. They are less appropriate when the intervention primarily affects outcomes not captured by generic preference based instruments (such as caregiver burden, process utility, or health states not well described by the EQ 5D dimensions). In pediatric populations, measuring utilities is methodologically challenging, and alternative approaches may be needed.

A common error is using utility weights from different instruments interchangeably, even though different instruments produce systematically different values for the same health state. This can lead to inconsistent QALY estimates across studies. Another mistake is failing to discount future QALYs or applying inconsistent discount rates for costs and outcomes. Researchers sometimes calculate QALYs using utilities derived from patient self report and assume they are equivalent to utilities from general population preferences, when the two can differ substantially (patients often rate their own health states higher than the general public would). Finally, treating all QALYs as equal regardless of who gains them (a principle called QALY egalitarianism) has been criticized on equity grounds, but departures from this principle require explicit justification.

A cost effectiveness analysis compares a new surgical procedure against medical management for severe obesity. Over a 10 year time horizon, patients receiving surgery gain an average of 1.5 additional QALYs compared to medical management (discounted at 3.5% per year), primarily through improved utility weights (from 0.62 to 0.79 on the EQ 5D, worth about 1.4 to 1.5 discounted QALYs over the 10 years) and a modest survival benefit. The surgery costs an additional $22,000 over the same period, yielding an ICER of $14,667 per QALY (22,000 divided by 1.5). At the willingness to pay threshold of $50,000 per QALY used in the analysis, the surgery is considered cost effective.