Why diagnosis matters

Why diagnosis matters.

In this section, we will learn about the role of diagnostics in the detection of infection. First, what are diagnostics? A diagnostic test for an infectious disease can be used to demonstrate the presence or absence of infection or to detect evidence of a previous infection– for example, the presence of antibodies. Demonstration of the presence of the infecting organism or a surrogate marker of infection is often vital for effective management and for guiding treatment of infectious diseases. Basic diagnostic methods for infectious diseases typically fall into two different categories. The first category is pathogen detection. One example of this is a molecular assay that can detect the DNA or RNA of an infectious organism in order to be able to identify it.

In another example, antigen detection assays can detect proteins present in an infectious organism. The second category of diagnostic methods are host biomarker detection tests. The most common example of this is an antibody detection test that can detect, for example, an IgG antibody or an IgM antibody, which are two types of antibodies that the body produces in response to an infection. In terms of characteristics of diagnostic tests, to be useful, they must be accurate and fit for use in the population and setting for which they are intended. We measure the accuracy of a diagnostic test via the following characteristics.

First, sensitivity, which is the percentage of truly infected individuals who will test positive on a test; and second, specificity, which is the percentage of truly uninfected individuals who test negative. Sensitivity and specificity are usually determined against a reference standard test, which is sometimes referred to as a “gold standard”, which is used to determine which subjects are truly infected when evaluating a new test. Additional characteristics of diagnostic tests that are important to consider are positive predictive value, or PPV, and negative predictive value, or NPV. Positive predictive value is defined as the percentage of those testing positive by a test who are truly infected.

Put another way, if you have someone who tests positive, the positive predictive value is what percent chance does that person actually have the infection. Negative predictive value is defined as the percentage of those testing negative by a test who are truly uninfected. Both PPV and NPV depend not only on the sensitivity and specificity of the test but also on the prevalence of infection in the population studied. When we consider the role of diagnostics in the detection of infection, there are many important goals to consider. One consideration is having the right test, and a key question to consider with this goal is whether the test you’re using is appropriate for the clinical setting. Another important factor is testing the right patient.

And here we think about: will the clinical care of the patient be affected by the test result? Using the test at the right time is also critical.

Here, important questions are: will the result be available in time to inform care of the patient or public health actions, such as quarantine? And will the clinician act upon the test result promptly? Understanding the right interpretation of the test is also important.

And here we think about: will the clinician understand the test result? And finally, ensuring that the right treatment and patient management decisions are based on that test result. Will further testing be needed to confirm the diagnosis? Diagnostics often do not exist, are inaccessible, or cost too much. They remain under-resourced and overlooked, despite being pivotal for guiding the best treatment for individual patients, for preventing the spread of disease, and for enhancing surveillance for early disease detection and monitoring. Undiagnosed patients can unknowingly transmitted disease to others. In the case of COVID-19, most estimates of the incubation period range from one to 14 days, most commonly around five days.

In this time, patients may be asymptomatic and unaware they are infected, and much of the transmission may occur during this period. A quick and accurate diagnosis can ensure that the right care and treatment is administered to the patient in time, allowing for more effective treatment. In the case of COVID-19, in the absence of either proven effective therapy or a vaccine, diagnostic testing is the most valuable tool we currently have in order to isolate and quarantine patients to reduce transmission, as well as target supportive treatment to patients who are severely ill. The World Health Organisation has stated that diagnostic testing, isolation, and contact tracing should be the backbone of the global response to COVID-19.

FIND is working in partnership with WHO to accelerate development and access to diagnostics as part of the global response to the COVID-19 pandemic. I hope you have no doubt in your minds about the importance of diagnostics, especially in the case of this pandemic.