At the end of 2019, the first reports of a new respiratory virus appeared in China. The subsequent COVID-19 pandemic has affected every person, in every country, in the world. One early lesson was the crucial importance of timely accurate diagnosis. A second lesson was the widespread scarcity of such diagnostic capacity and capability.

The second lesson supported the findings of the 2018 Lancet Series on Pathology and Laboratory Medicine in Low-Income and Middle-Income Countries, namely that despite diagnostics being central to health care, access to diagnostic testing in pathology and laboratory medicine (PALM) is poor and inequitable in many parts of the world. In diagnostic imaging (DI), the other major diagnostic discipline, data are scarce, but what data are available suggest the situation is similar or even worse.

Poor accessibility of diagnostics is not a new issue. In 2008, the Maputo Declaration on Strengthening of Laboratory Systems identified the need to address the problems of poor accessibility to diagnostic testing. Although progress has been slow, there is now a conjunction of factors that has the potential to accelerate change. First, three major global health priorities—universal health coverage, antimicrobial resistance, and global health security—all require better access to diagnostics. Second, the publication of an essential diagnostics list (EDL) for priority settings by WHO in 2018 has been a key step in recognising the importance of diagnostics. Third, the COVID-19 pandemic has greatly raised awareness of the crucial importance of diagnostics. Lastly, within the past 15 years, extraordinary innovations in technology and informatics promise transformation across all aspects of diagnostics. The combination of all these factors can fuel political will to accelerate change.

This Lancet Commission on Diagnostics was set up with the remit of analysing the issues and identifying solutions for both PALM and DI, in part because these are the two major diagnostic disciplines and in part because, increasingly, optimum patient care (eg, in cancer) depends on the integration and synthesis of the results of both disciplines. Also, both disciplines share many of the same issues; for example, insufficient financial support, staff shortages, infrastructure problems, and low visibility and, hence, low priority.

In this Commission, we analyse the current status of diagnostics with the use of the six WHO building blocks of health systems, namely health service delivery, health workforce, health information systems, access to diagnostics (analogous to essential medicines), financing, and leadership and governance, as the basis. Given the dearth of reliable and comprehensive data, the Commission's first step was to quantify, where possible, the current state of diagnostics globally. We use six tracer conditions (diabetes, hypertension, HIV, and tuberculosis in the overall population, plus hepatitis B virus infection and syphilis for pregnant women) and show that the diagnostic gap (ie, the proportion of the population with the condition who remain undiagnosed) is, at 35–62%, the single largest gap in the care pathway (the cascade of care comprising screening, diagnosis, treatment, and cure or successful management). We also examine the current availability of diagnostics by level of health care facility, geography, and socioeconomic group. The diagnostic gap is most severe at the level of primary health care, in which only about 19% of populations in low-income and lower-middle-income countries have access to the simplest of diagnostic tests (other than those for HIV and malaria). Even in hospitals, this figure only rises to 60–70%. DI is essentially absent outside of hospitals. People who are poor, marginalised, young, or less educated have the least access to diagnostics.