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Rapid Responses: Rapid Responses published:
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rK39 strip test; Iranian experience
- Abdolvahab Alborzi, Behrooz Astaneh (2 August 2006)
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Re: rK39 strip test; Iranian experience
- François Chappuis, Suman Rijal, Alonso Soto, Joris Menten, and Marleen Boelaert (4 August 2006)
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Tests for visceral leishmaniasis and other conditions
- Huw Llewelyn (9 October 2006)
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Re: Tests for visceral leishmaniasis and other conditions
- Francois Chappuis, Suman Rijal, Alonso Soto, Joris menten, Marleen Boelaert (21 November 2006)
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Abdolvahab Alborzi, Head of Prof Alborzi Clinical Microbiology Research Center Prof Alborzi Clinical Microbiology Research Center, Nemazee hospital, Shiraz, Iran,, Behrooz Astaneh
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Dear Sir Regarding your valuable article, I would like to draw your attention to the article entitled: “Evaluation of rK39 strip test for the diagnosis of visceral leishmaniasis in infants” that is a report from Iran and has published in Eastern Mediterranean Health Journal Vol 12 No 3/4, 2006. Comparing the results of your met-analysis with our report, there are some interesting points that can be considered: -In the article, we reported 100% specificity for the test that differs
from yours, which is 90.6%. But the sensitivity of the test in our report
was 82.4%.
We agree with you that using rK39 is a suitable test for the diagnosis of visceral leishmaniasis. Besides we propose that this test is a very reliable to use in those developing countries where there is no access to laboratory facilities. Sincerely yours
Competing interests: None declared |
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François Chappuis, lecturer Travel and Migration Medicine Unit, Geneva University Hospitals, 1211 Geneva 14, Switzerland, Suman Rijal, Alonso Soto, Joris Menten, and Marleen Boelaert
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Dear colleagues, We thank you for the valuable comments. We agree that the rK39 dipstick needs to be evaluated for the diagnosis of visceral leishmaniasis in Leishmania infantum endemic areas. Only one of the thirteen studies included in our meta-analysis was conducted in a L. infantum endemic country. The 100% specificity found in your study should not be compared with the 90.6% specificity reported in our meta-analysis as this result comes from the analysis of the 4 studies that prospectively included patients with clinical suspicion of visceral leishmaniasis. As the controls included in your study were patients with other diseases coming from a non -endemic area, this most likely lead to an overestimation of the specificity. Actually, your results match well the 97.1% specificity found in our meta-analysis of the 7 studies that included patients with cross- reacting diseases as controls (table 1). The rK39 dipstick had a relatively low sensitivity in your study (82.4%) but we must emphasize that because of the low number (17) of cases of visceral leishmaniasis that were included, the margins of the 95% confidence interval are very wide (59.0 – 93.8%). This is indeed of interest that your study was conducted in infants and young children, the main target of L. infantum-induced visceral leishmaniasis in Iran. As the results of the dipstick diagnostic performance was not stratified by age in any of the studies included in the meta-analysis, we could not proceed with the analysis of age sub- groups. Sincerely yours Competing interests: None declared |
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Huw Llewelyn, Consultant Physician Great Western Hospital, Swindon, SN3 6BB
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The specificity of the direct agglutinin test (DAT) and the rK39 dipstick tests vary depending on how many healthy individuals were present in the different papers’ control groups [1]. If a test is to be used alone for screening asymptomatic patients, then it is valid to include apparently healthy patients when assessing ‘specificity’. However, if a test is to be used for screening patients who already have symptoms e.g. fever or weight loss, then it is not appropriate to calculate ‘specificities’ based on apparently healthy subjects. The inclusion of healthy patients in a study to assess diagnostic tests may falsely inflate the ‘specificity’ and thus the ‘likelihood ratio’. The DAT and the rK39 dipstick tests were positive in as many as 9.4 to 17.4% of symptomatic patients without leishmaniasis. It would be helpful for diagnostic purposes to know how frequently each of these other conditions (e.g. malaria, TB, or a self-limiting illnesses) produced positive DAT or rK39 results. These other conditions would thus be the ‘differential diagnoses’ of the positive DAT and rK39 dipstick test results. It would also be helpful to know how often each of these ‘differential diagnoses’ occurred in patients with positive results. It is important to recognise that two different approaches can be used when assessing diagnostic tests. One is the ‘screening’ approach, which uses Bayes theorem to calculate the probability of one diagnosis in a population by using the indices of sensitivity and specificity. The other approach involves considering differential diagnoses suggested by a diagnostic lead [2]. This ‘diagnostic lead’ approach does not use ‘specificities’ at all but instead uses the frequency of occurrence of findings in each differential diagnosis (their ‘sensitivities’) and also the frequency of each differential diagnosis in those with a diagnostic lead. References 1. Chappuis F, Rijal S, Soto A, Menten J, Boelaert M. A meta- analysis of the diagnostic performance of the direct agglutination test and rK39 dipstick for visceral leishmaniasis. BMJ 2006; 333:723, 723-6. 2. Llewelyn H, Ang H A, Lewis K, Al-Abdullah A. The Oxford Handbook of Clinical Diagnosis, Oxford University Press, 2006. Competing interests: None declared |
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Francois Chappuis, Lecturer Geneva University Hospital, 1211 Geneva 14, Suman Rijal, Alonso Soto, Joris menten, Marleen Boelaert
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Dear Colleague, We thank you for your interesting comments. We agree that studies that include healthy individuals as controls inflate the specificity of diagnostic tests under evaluation. This is the reason why we performed separate meta-analyses of the diagnostic performance of the DAT and the rK39 dipsticks in various control groups and underlined the results of studies conducted in patients with clinically suspected visceral leishmaniasis (VL). Following your letter, the results of the DAT and the rK39 dipstick in patients with diseases other than VL have been revised and are available on request. More extensive data is available for the DAT than the rK39 dipstick. Positive results appear to be more common in infections caused by closely related organisms such as Trypanosoma cruzi (2% with DAT and 18% with rK39 dipstick) or Trypanosoma brucei spp (18% with DAT and 20% with rK39 dipstick) and are thus likely to be true false positive results. Positive results observed with diseases such as malaria (2% with DAT and 7% with rK39 dipstick) or bacterial infections (7% with DAT and 10% with rK39 dipstick) may also be false positive results, in particular if these patients come from a VL endemic area, thus carrying a significant risk to have been infected with Leishmania donovani sl in the past. However, as it is common for these diseases to occur concomitantly with VL, a subset of these patients were possibly suffering of VL unrecognised by the test(s) of reference used during the study. In this situation, the positive test result would thus be a true positive result. Competing interests: None declared |
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