Mexico

evaluating diagnostics

A guide for diagnostic evaluations
Rosanna W. Peeling, Peter G. Smith and Patrick M. M. Bossuyt

Abstract | Accurate diagnostic tests have a key role in patient management and the control of most infectious diseases. Unfortunately, in many developing countries, clinical care is often critically compromised by the lack of regulatory controls on the quality of thesetests. The information available on the performance of a diagnostic test can be biased or flawed because of failings in the design of the studies which assessed the performance characteristics of the test. As a result, diagnostic tests are sold and used in much of the developing world without evidence of effectiveness. Misdiagnosis leading to failure to treat a serious infection or wasting expensivetreatment on people who are not infected remains a serious obstacle to health.
With these supplements, we aim to provide simple, user-friendly operational guides on how to design and conduct evaluations of diagnostic tests for infectious diseases that are of public health importance in the developing world. Each guide will contain a set of general principles on the design and conduct ofdiagnostic evaluations followed by disease-specific considerations. The first in this series was the malaria guide. This article provides background information and discusses why such guides are needed and their importance in improving the diagnosis of infectious diseases in the developing world.
The need for good qualiTy diagnosTic TesTs The lack of access to good quality diagnostic tests for infectiousdiseases contributes to the enormous burden of ill health in the developing world, where infectious diseases are the major causes of death and account for more than half of all deaths in children1 (Table 1, FIG. 1). Each year, more than 2 million people die of malaria, approximately 4 million of acute respiratory infections and almost 3 million of enteric infections. HIV and tuberculosis togetherare estimated to kill some 5.8 million people each year2,3. More than 95% of these deaths are in developing countries. Early diagnosis and treatment not only reduces the risk of the patient developing long-term complications but for diseases
s2 | nOVEMbEr 2008

such as tuberculosis, sexually transmitted infections (STIs) and HIV, prompt treatment also reduces further transmission of the diseaseto other members of the community. A confident diagnosis can sometimes be made on the basis of clinical signs or symptoms but accurate diagnosis usually requires a specific diagnostic test, often involving access to a diagnostic laboratory. In settings where access to diagnostic laboratory services is limited, the WHO recommends the use of a syndromic approach to clinical management, wherepatients presenting with a particular syndrome are treated for all of the major causes of the syndrome. Algorithms for syndromic management have been developed for STIs and for common childhood diseases, the latter through the integrated management of childhood illness (IMCI)4,5. Although such algorithms are simple to use and the recommended treatment packages are generally

inexpensive, a majordisadvantage of this approach is the risk of giving inappropriate treatment to people without the syndromically diagnosed disease and the accompanying potential for inducing antibiotic resistance. Diagnostic tests can complement syndromic management by facilitating evidence-based management of patients, improving the specificity of treatment and, in some diseases, allowing contact tracing and otherdisease-control measures. Laboratory testing is perhaps most useful for detection of asymptomatic infections to prevent development of sequelae and transmission, and for public health surveillance and interventions. Table 2 shows the role of diagnostic tests in the control of some of the diseases that are prevalent in developing countries6. Good quality diagnostic tests that are fit for purpose and...