Patients with multiple conditions are the rule rather than the exception in primary care.¹ In a recent study of 21 family practices in the Saguenay region, Quebec, the prevalence of multimorbidity was 69% in 18-44 year olds, 93% in 45-64 year olds, and 98% in those aged over 65, and the number of chronic conditions varied from 2.8 in the youngest to 6.4 in the oldest.¹ Other countries report a similar burden.^{2 3} The number of Americans with multimorbidity is estimated to rise from 60 million in 2000 to 81 million by 2020.⁴

Having multiple chronic medical conditions is associated with poor outcomes: patients have decreased quality of life,⁵ psychological distress,⁶ longer hospital stays, more postoperative complications, a higher cost of care, and higher mortality. Multimorbidity also affects processes of care and may result in complex self care needs⁷; challenging organisational problems (accessibility, coordination, consultation time); polypharmacy; increased use of emergency facilities; difficulty in applying guidelines; and fragmented, costly, and ineffective care.

Yet most research and clinical practice is still based on a single disease paradigm which may not be appropriate for patients with complex and overlapping health problems. Classic clinical trials tend to emphasise efficacy at the expense of effectiveness. In doing so, they exclude patients with multiple conditions, thereby compromising the external validity and the relevance of the trials for this population.⁸

Research on multimorbidity is in its infancy.⁹ So far, most research has investigated the epidemiology of multimorbidity, its effect on physical functioning, and its measurement. Much less studied is the effect of multimorbidity on processes of care and what constitutes “best care” for these patients.

Areas for potential investigation of multimorbidity fall primarily into three categories—defining and categorising the population; developing the tools needed to explore multimorbidity and its consequences; and using these tools to investigate promising processes of care.

Who are the patients with several conditions? What is their risk profile? How do we distinguish multimorbidity from related concepts such as complexity, frailty, and polypharmacy? How do we classify multimorbidity and comorbidity in terms of conditions that need disparate versus congruent treatment strategies? For example, how does the patient with coronary disease, hypertension, and diabetes differ from the one with pulmonary disease, arthritis, and depression? In which situations is a subjective or an objective measure of multimorbidity more appropriate? Investigators have begun to look at several of these complex questions, but standards have not yet been developed.¹⁰

The results of prevalence studies reveal a complex picture of coexisting diseases. We now require a clear conceptual framework that includes consistent measures of multimorbidity and permits comparisons between studies. This will facilitate the next step—investigating improved processes of care. What are the best processes for making decisions in the context of multiple, often ill defined, problems and fragmentary evidence?¹¹ How should we assess the shifting priorities of patients and providers, design adaptive responses to unpredictable aspects of the illnesses, and organise multiple resources to achieve specific health goals?¹¹ What affects processes of care, and what constitutes best care? Which outcomes matter to these patients in which situations? How do we implement whatever best care turns out to be?

Answers to these questions will require continual experimentation, with substantial innovation and reform in healthcare delivery and organisation. Models of collaborative, patient centered, and goal oriented care are more likely to meet the complex needs of patients with multimorbidity. Involving patients in the research process and making good use of mixed methods research designs that incorporate both patient and provider perspectives may also help answer complex clinical questions.

The study of multimorbidity is particularly appropriate for the international research community for several reasons. Research is in its infancy, and appropriate collaboration may minimise redundancy and promote efficient and timely research. Different international communities have varied access to administrative data that can be used to paint broad pictures of caring for people with several conditions. The World Health Organization has given priority during the next decade to worldwide prevention and care of chronic illness.¹² International collaboration specifically among primary care researchers may result in patient centered and low tech care practices that can be translated into practice in varied settings and across different healthcare systems.

As a step towards facilitating this collaboration, we have started a virtual research community to discuss research questions specifically directed towards international communication on multimorbidity (www.med.usherbrooke.ca/cirmo/). The increasing number of primary care research networks in many countries also offers an ideal setting for collaboration to occur. The time has come not only to include people of all ages with multimorbidity in research efforts, but to focus on improving the care of this vulnerable and growing population.