Multimorbidity has become a norm rather than the exception in primary care. It is most frequently defined as the co-existence of 2 or more chronic diseases within an individual. Its prevalence is highest among the elderly population, making this problem a rising and urgent public health concern in ageing societies. Poor quality of life is together with disability, functional decline and high health care costs a major consequence of multimorbidity. It is also a valuable measure for evaluation of health care services. The current study aims to:

–understand the relation between the number of diseases and quality of life through systematic literature review;

–explore the strength of this association by conducting a first meta-analysis on the subject.

By following PRISMA protocol, four databases were searched: PubMed, Embase, CINAHL and PsycINFO, collecting the studies published until December 2017. Only original studies with clear operationalisation of multimorbidity and validated quality of life measures were retained for systematic review, while meta-analysis was performed on minimum of three studies with the same quality of life measurement. References with index disease or disease patterns were not included. For meta-analysis the regression slopes of the association between an increasing number of diseases and quality of life were used. Random effect model was fitted for obtaining a pooled effect estimate. Sources of heterogeneity were explored by performing meta-regression.

Out of 18073 titles, 71 studies were kept for systematic review, of which 42 were included in meta-analysis. Preliminary results showed that a higher number of diseases were associated to a lower quality of life. The decrease in quality of life per each added disease, depending on the quality of life scale used was as follows: for EQ-5D scale [−4.05 (95% CI: −5.22; −2.89)], for physical component summary of pooled SF-36, -12 and -8 scales [−2.51 (95% CI: −3.02; −2.01)] and for mental component summary of pooled SF-36, -12 and -8 scales [−0.81 (95% CI: −1.07; −0.54)]. Heterogeneity between the studies was high (96.4%, 99.9% and 93.3%).

The co-existence of an increasing number of chronic diseases is associated with decreasing quality of life. Additional studies, which consider the length and severity of diseases are required for clarifying further this association. That will allow a better understanding of the impact that multiple conditions have on patient's quality of life and facilitate effective individually tailored interventions among multimorbid patients. Standardisation of multimorbidity measurement and quality of life tool would enable better comparability between studies.