Performance characteristics and quality control of community based ultrasound surveys for cystic and alveolar echinococcosis.
The probability of disease given the results of a test, is called the predictive value of the test. The predictive value of a test is not a property of the test itself but will vary according to the prevalence of the disease in the studied population. The positive predictive value (PPV) is the probability that the subject tested has the disease given that a positive result is obtained. The negative predictive value (NPV) is the probability that the subject tested is normal given that a negative result is obtained. As the prevalence of a disease in a population approaches zero so does the PPV and most of the positive cases will be 'false positives'. Conversely the NPV will be very high at low prevalences and there will be few 'false negative' results. The sensitivity and specificity of a test are properties of the test and do not vary with prevalence. The higher the sensitivity and specificity of a particular test the greater the predictive values will be at any given prevalence of the disease. Ultrasound (US) is increasingly used for detecting lesions due to cystic and alveolar echinococcosis (CE and AE) and portable US scanners facilitate community based mass screening surveys in remote rural communities. Screening is justified with AE and CE in endemic areas as diagnosis at an early stage can lead to a better prognosis following treatment. The sensitivity and specificity of US has been reported to be between 88-98% and 95-100% respectively for CE and the sensitivity is a little higher for AE. Both species have pathognomonic signs on US and the technique is considered to be the 'gold standard' although it is still an imperfect test. Clinical, laboratory and epidemiological data also play an important role in the diagnosis of CE and AE. US results where possible, should be evaluated in relation to these findings. Suspected CE and AE images, may benefit from the use of other imaging techniques such as magnetic resonance imaging, computerised tomography and in the case of AE angiography or cholangiography. Immunological tests or molecular biological techniques also provide a useful back up, especially for AE. As sensitivity and specificity are properties of the US diagnostic test they should not vary if the case mix reported in different studies remains the same. The use of the WHO standardised US classifications for CE and AE should be used so that the properties of the test are standardised. Quality control of field based studies will depend on geographical variations in the case mix and the relative proportions of cyst types without pathognomonic signs. The latter will have the most bearing on variations in specificity, as would the use of different classifications. Inter- and intra-observer variability and differences in prevalence will affect the performance of US in different endemic settings. Community based surveys must adhere to the highest ethical standards and the outcome of surveys should result in appropriate treatment and follow-up strategies for all infected individuals and suspected cases found during the surveys.
Macpherson
Calum N L
CN
Windward Islands Research and Education Foundation, St. George's University, P.O. Box 7, St. Georges, Grenada, West Indies. cmacpherson@sgu.edu
Milner
Ruth
R
D000818:Animals; D004443:Echinococcosis / Q000175:diagnosis / Q000000981:diagnostic imaging*; D004444:Echinococcosis, Hepatic / Q000175:diagnosis / Q000000981:diagnostic imaging*; D004446:Echinococcus / Q000254:growth & development / Q000302:isolation & purification; D006306:Health Surveys; D006801:Humans; D008403:Mass Screening; D011237:Predictive Value of Tests; D015995:Prevalence; D011786:Quality Control; D012111:Residence Characteristics; D012307:Risk Factors; D012680:Sensitivity and Specificity; D014463:Ultrasonography / Q000592:standards; D014944:World Health Organization