Quality-Adjusted Life Year (QALY)

Quality-Adjusted Life Year (English: Quality-Adjusted Life Year, abbreviated as QALY) is a unit used to measure the outcome of a health intervention or health condition. This metric combines changes in both the duration of life (quantity) and the quality of that life (quality) into a single index. QALY was developed particularly in the field of health economics to assist decision-making processes regarding how limited resources should be allocated among different health programs and technologies.

Definition and Calculation

QALY is calculated by multiplying the length of time a person spends in a specific health state by the quality-of-life weight assigned to that state. In this calculation, quality of life is typically rated on a scale from 0 to 1. On this scale, a value of 1 represents perfect health, while a value of 0 represents death. In some cases, negative values may also be used for health states considered worse than death.

One QALY is equivalent to one year lived in perfect health. For example, a person who lives for 10 years in a health state rated at a quality-of-life index of 0.8 has a total QALY value of 8 (10 years × 0.8). The benefit provided by a health intervention is expressed as a “QALY gain.” This gain reflects the difference in both quality of life and life expectancy before and after the intervention. An example calculation can be performed as follows:^【1】 :

• If a person has a quality-of-life index of 0.6 before a surgical intervention and it increases to 0.9 after the intervention, the treatment provides a gain of 0.3 QALY per year (0.9 - 0.6).
• If the person’s life expectancy after treatment is 10 years, the total QALY gain is 3 (0.3 × 10).

This calculation method aims to provide a common basis for comparing different diseases, patient groups, and treatment regimens.

Historical Development and Theoretical Foundations

The emergence of the QALY concept is closely linked to changes in the understanding of health. The World Health Organization’s (WHO) 1946 definition of health not merely as the absence of disease or infirmity but as “a state of complete well-being in physical, mental, and social aspects^【2】 ” elevated the importance of quality of life in health assessments. As life expectancy increased and chronic diseases became more prevalent, the goals of medical interventions expanded to include not only extending life but also improving its quality. In this context, a standardized unit capable of measuring the efficiency of different health outcomes became necessary.

The theoretical foundation of the QALY approach rests on neoclassical welfare economics. According to this theory, under a fixed health budget, the objective is to maximize the total health benefit to society, measured in QALYs. The validity of this model depends on a set of assumptions.^【3】 :

1. The health budget is fixed.
2. The sole objective is to maximize health benefits in QALY terms across the population.
3. Total information is available on the cost and effectiveness (ICER) of all interventions.
4. Health programs are perfectly divisible (can be allocated in any fractional amount).
5. Programs exhibit constant returns to scale (i.e., expanding the program increases costs and effects proportionally).

In practice, it is generally impossible to satisfy all these assumptions, which leads to practical limitations and criticisms of the QALY approach.

Applications and Methods

QALY is most commonly used in health technology assessments and resource allocation decisions.

Cost-Utility Analysis and Resource Allocation

QALY forms the basis of cost-utility analyses. In these analyses, the Incremental Cost-Effectiveness Ratio (ICER) is calculated to determine how cost-effective one health intervention is compared to another (or to no intervention). ICER is calculated by dividing the difference in cost between two interventions by the difference in QALY gain between them.^【4】 :

<span class="katex"><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.6833em;"></span><span class="mord text"><span class="mord">ICER</span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1.3355em;vertical-align:-0.4033em;"></span><span class="mord"><span class="mopen nulldelimiter"></span><span class="mfrac"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.9322em;"><span style="top:-2.655em;"><span class="pstrut" style="height:3em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord text mtight"><span class="mord mtight">Etkililik A</span></span><span class="mbin mtight">−</span><span class="mord text mtight"><span class="mord mtight">Etkililik B</span></span></span></span></span><span style="top:-3.23em;"><span class="pstrut" style="height:3em;"></span><span class="frac-line" style="border-bottom-width:0.04em;"></span></span><span style="top:-3.4461em;"><span class="pstrut" style="height:3em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord text mtight"><span class="mord mtight">Maliyet A</span></span><span class="mbin mtight">−</span><span class="mord text mtight"><span class="mord mtight">Maliyet B</span></span></span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.4033em;"><span></span></span></span></span></span><span class="mclose nulldelimiter"></span></span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.8889em;vertical-align:-0.1944em;"></span><span class="mord text"><span class="mord">QALY ba</span><span class="mord accent"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.4306em;"><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="mord">s</span></span><span style="top:-3em;"><span class="pstrut" style="height:3em;"></span><span class="accent-body" style="left:-0.2222em;"><span class="mord">¸</span></span></span></span><span class="vlist-s">​</span></span><span class="vlist-r"><span class="vlist" style="height:0.1701em;"><span></span></span></span></span></span><span class="mord latin_fallback">ı</span><span class="mord"> maliyet</span></span></span></span></span>

The resulting ICER value is compared with a pre-determined "payment threshold." If the cost per QALY of an intervention is below this threshold, the intervention is considered "cost-effective." Institutions such as the National Institute for Health and Care Excellence (NICE) in the United Kingdom use such threshold values as a reference in reimbursement decisions. The World Health Organization has suggested that one to three times a country's gross domestic product per capita may serve as a potential threshold range. However, the use of a fixed threshold value and how it should be determined remain subjects of significant debate, particularly across different health systems such as fixed-budget Beveridge-type systems versus unfunded Bismarck-type systems.

Measuring Quality of Life

Quality of life weights (utility values) required for QALY calculations are obtained through various methods. These methods are divided into two categories: "direct" and "indirect."

Direct Measurement Techniques

These techniques directly measure individuals' preferences.

• Standard Gamble: The individual is asked to choose between continuing to live in their current health state or taking a gamble that offers either perfect health or immediate death, with probabilities assigned to each outcome. The probability of death at which the individual becomes indifferent between the two options determines the utility value of that health state.

• Time Trade-Off: The individual is asked to choose between living a longer period with a specific health problem or living a shorter period in perfect health. The amount of time the individual is willing to trade off indicates the quality of life value.

Indirect Measurement Techniques

Multi-dimensional questionnaires are used to describe individuals' health states. The health profiles derived from these questionnaires are converted into a single QALY score using previously established sets of utility values (algorithms) obtained from general population samples. Commonly used indirect scales include EQ-5D,SF-6DandHealth Utility Index (HUI)^【5】  bulunur. EQ-5D evaluates five dimensions: mobility, self-care, usual activities, pain/discomfort, and anxiety/depression.

Alternative Approaches and Criticisms

Although presented as a tool for the rational allocation of health resources, QALY has faced numerous theoretical and ethical criticisms. Criticisms generally focus on its utilitarian foundations, concerns about equity and discrimination, and the narrow scope of value judgments.

Utilitarian Foundation

QALY maximization is based on a utilitarian approach that seeks to achieve the greatest good for the greatest number. Ethicists argue that this approach may overlook other moral values such as individual rights and justice.

Equity and Discrimination Concerns

• Ageism: QALY calculations tend to assign higher value to interventions that save the lives of younger individuals, who have longer life expectancies. This is interpreted as discrimination against older patients. Counterarguments such as the “fair innings” argument assert that every individual has a right to a certain span of life and that priority should be given to those who have not yet lived that span.

• Disability and Chronic Illness: QALY assigns lower values to the highest achievable health state of individuals with chronic illness or disability compared to “perfect health” (value of 1), potentially devaluing interventions that save their lives. This has led to criticism that QALY discriminates against people with disabilities.

• Other Factors: Some criticisms suggest that QALY may indirectly create a basis for discrimination based on the patient’s lifestyle (e.g., diseases caused by smoking), social role (e.g., having dependent children), or gender.

Narrow Scope of Value Judgments

• Initial Severity of Illness: QALY focuses only on the “change” in health status and does not directly account for the severity of the health condition before intervention. However, public opinion surveys show that people often prioritize helping those in the worst condition—even if the health gain is small—following the “rule of rescue.”

• Distribution of Benefits: The QALY approach is indifferent to how health gains are distributed (“distributional neutrality”). For example, it makes no distinction between allocating 100 QALYs to 100 people (1 QALY each) or to 2 people (50 QALYs each). Yet society may often prefer that benefits be spread more widely.

Alternative and Enhanced Models

Criticisms of QALY have led to the development of alternative or complementary models. Some of these include:

• Weighted QALYs: Models that propose assigning greater weight to QALYs gained by specific patient groups (e.g., severely ill patients, children) to address equity concerns.

• Cost-Value Analysis: Approaches that explicitly incorporate factors such as disease severity or the patient’s potential for benefit, offering a broader value framework than QALY.

• Disaggregated Presentation of Outcomes: Some experts advocate presenting cost and outcome data separately (e.g., years of life gained, improvement in quality of life, patient financial burden) rather than combining them into a single QALY or ICER value. This “cost-consequence” approach allows decision-makers to weigh different values according to their own priorities.