As described above, a CUA is frequently the recommended type of economic evaluation and the outcome that is suggested for use by all contemporary guidelines is the QALY. A QALY is calculated by multiplying the utility (a measure of preference for a person’s overall quality of life – usually varying on a scale from 0 to 1, where 1 represents perfect health and 0 is equivalent to death) for a given health state by the time spent in that health state [
14]. Utility scores can be determined in a variety of ways. They can be measured directly – through use of the visual analogue scale, the time trade-off and the standard gamble [
14], or they can be measured indirectly, through questionnaires like the Euroqol EQ-5D, or the Health Utilities Index.
In the visual analogue scale participants with a certain health condition are presented with a scale ranging from worst to best imaginable health state and on that scale they place where they feel their current health state is [
15]. This provides subjective weights and an ordinal ranking of health outcomes, but it does not invoke the notion of trade-off [
15]. In the time trade-off method individuals have a choice between living the rest of their life (
t) in a particular health state (
i) or living for a shorter time period (
x) but in perfect health. The time is varied until the participant feels ambivalent about the two options and then the preference score for
i is
x/
t [
15]. In the standard gamble individuals have to choose between the certainty of remaining in a given health state
i and an alternative with two outcomes of perfect health with the probability
p and death with the probability 1-
p. The
p is varied until the participant is indifferent between the two choices and then the score for state
i for time
t is
p [
15]. If a participant places a higher value on state
i then a higher probability of perfect health will be needed for the individual to be indifferent between
i and the gamble of having perfect health [
15]. In both cases a score between 0 and 1 results, with higher scores reflecting better overall quality of life.
As the above direct methods can be time consuming and challenging for patients, several instruments have been developed that can generate utilities from scores on a variety of domains, for instance the EQ-5D, the SF-6D and the health utilities index [
14]. These generic measures are used for valuing health related quality of life based on health status within certain areas [
14]. The responses to these questionnaires are then converted into a single utility value.
We illustrate the use of QALYs with an example from the IPC literature which compared different IPC programs designed to prevent surgical site infections [
16]. The authors used hypothetical data to estimate how surgical site infections would impact morbidity and mortality and the influence of these infections could be measured in QALYs. Two different scenarios were represented, in the first scenario patients either did or did not develop an infection and those who did develop an infection died shortly after surgery. The patients who did not develop an infection had 7.575 more QALYs than those who did acquire an infection. In another scenario, a patient develops an infection but recovers and after several months improves to the same health state as a patient who never develops an infection. This patient has 7.475 QALYs after surgery [
16].