Diagnostic efficacy of Widal slide agglutination test against Widal tube agglutination test in enteric fever

Abstract

Introduction: Enteric fever is an endemic disease in India and warrants rapid and affordable diagnosis. The Widal slide agglutination test is a commonly used rapid screening test for this purpose. The literature available on its diagnostic ability in comparison to the tube agglutination test is however scanty. Hence, this study aims to evaluate the effi cacy of the Widal slide agglutination test and the tube agglutination test for the diagnosis of enteric fever. Materials and Methods: A total of 1470 sera were collected during the study period of one year from patients having pyrexia of unknown origin. All the samples were tested for the presence of anti O and anti H agglutinins against S. typhi and S. paratyphi A by semi quantitative slide and quantitative tube agglutination tests as per standard protocols. The titers of 1:80 (O agglutinins) and 1:160 (H agglutinins) were taken as the signifi cant titer for the diagnosis of enteric fever. The results of the slide agglutination test were compared with the tube agglutination test and analyzed using Fisher's exact test. The sensitivity, specifi city, positive and negative predictive values of slide agglutination were calculated using the tube agglutination method as a standard for comparison. Results: Of the 294 slide positive samples, 209 (71.1%) samples tested negative by the tube agglutination test. The sensitivity, specifi city, positive and negative predictive values for the slide agglutination test were observed to be 100 % (CI 95.75-100%), 84.91% (CI 82.93-86.73%), 28.91% (CI 23.84-34.45%) and 100% (CI 99.69-100%) respectively. Conclusion: Serological diagnosis of enteric fever should always be confi rmed by the tube agglutination test rather than depending solely upon the rapid slide agglutination test results.

Comparative evaluation of slide agglutination and Widal tube agglutination test in detecting enteric fever among patients attending a tertiary care hospital in North India

“…Similar results of high false positivity and low PPV of slide agglutination (65.5% and 28.9% respectively) were also detected in various earlier done studies. 1,8 This clearly proves that, although the slide agglutination test is useful in screening out negative samples, the positivity expressed by it is not always helpful in diagnosing the disease correctly. The high false positivity exhibited by slide agglutination in the present study could be attributed to the endemicity of the disease in our region and possible presence of cross reacting antibodies of other bacterial and non-bacterial infections.…”

“…This is found to be similar to another study which showed sensitivity and specificity of slide test to be 100% and 84.9% respectively. 8 Another study showed sensitivity and specificity of slide test to be 53-80% and 57-90% respectively. 16 It has been found that the positive predictive value (PPV) is the most important measure of a clinical diagnostic method since it represents the proportion of patients with positive test results that are correctly diagnosed.…”

“…This could lead to serious consequences by unnecessarily pressuring the normal gut flora to develop antibiotic resistance and thus further increasing patient's sufferings. 8 …”

“…The titres of ≥80 for "O" antigen and ≥160 for "H" antigen were taken as local cut off titres (baseline or significant titres) to determine positivity of both the slide agglutination and Widal tube agglutination test. 8 …”

“…7 A blood culture gives positive results in 73-97% cases, when the sample has been taken in the early course of disease prior the use of any antibiotics. 8,9 However, in developing countries like India, sensitivity of blood cultures is lower as patients usually visit the hospital late in the course of disease and frequently they have taken antibiotics as self-medication or upon unauthorized prescription before visiting the hospital. Other demerits of the test are its cost and relatively long turnaround time.…”