Diagnostic Value of Widal Test in the Diagnosis of Typhoid Fever: A Systematic Review

Introduction: Typhoid fever the common cause of morbidity and mortality especially in the developing countries where Widal test is routinely used as diagnostic tool to rule out the disease. The diagnostic ability of Widal test is debatable as the test method has a low sensitivity, specificity and positive predictive value (PPV). Therefore, reviewing articles across the world regarding the diagnostic value of Widal test is necessary.


Introduction
Typhoid fever is a systemic disease caused by Salmonella typhi and is the major cause of morbidity and mortality worldwide [1]. Reports by the World Health Organization revealed that about 21 million cases and >600,000 annual deaths from typhoid fever occur throughout the world. Developing nations share the highest burden due to rapid population growth, increased urbanization, and limited safe water and health systems [2,3].
Accurate diagnosis of typhoid fever at an early stage is important not only for etiological diagnosis, but also to identify individuals that may serve as potential carriers, who may be responsible for acute typhoid fever outbreaks. Additionally, the diagnosis of typhoid fever on clinical grounds is difficult, as the presenting symptoms are diverse and similar to those observed with other febrile illnesses. Serodiagnosis of typhoid fever has been attempted since the late nineteenth century by Widal and Secard. The test is based on demonstrating the presence of agglutinins (antibodies) in the serum of an infected patient, against the H (flagellar) and O (somatic) antigens of Salmonella enterica serotype typhi (S. typhi) [4][5][6][7][8].
Widal test relies on the demonstration of a rising titer of antibodies in paired samples 10 to 14 days apart. In typhoid fever, however, such a rise is not always demonstrable, even in blood culture-confirmed cases. In addition, interpreting the test has been such a problem that different cut-offs have been reported from different places which makes difficulty in patient management. Furthermore, the test has a low sensitivity, specificity and positive predictive value (PPV) [9][10][11][12] which may create over diagnosis of typhoid fever, patient dissatisfaction, inappropriate economic loss and drug resistance in particular.
In the developed nations, Widal test is no longer used as a diagnostic tool due to the low prevalence of typhoid, access to safe drinking water, better laboratory facilities to isolate the bacteria, and the low sensitivity and specificity of the Widal test [13]. This is not the fact in developing countries including Ethiopia where Widal test is routinely used to diagnose typhoid fever; although, the diagnostic value of the test has been debated. Hence, reviewing articles across the world regarding the diagnostic value of Widal test is necessary.

Study design and data source
Systematic review of the published literature of observational studies was conducted. Original studies providing data on the diagnostic value of Widal test were identified through a computerized search using databases of Medline/PubMed, Google Scholar, HINARI (Health Inter Network Access to Research Initiative) and manual search with detailed search-strategy and cross-checking of reference lists. The search terms used to search the database were diagnostic value, sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV) and test efficiency of Widal test. The data abstraction was performed from September, 2015-July, 2016.

Study selection
A systematic review was made on observational studies which were reported on the diagnostic value of Widal test in diagnosing typhoid fever. Eligibility criteria for articles to be included in the review were articles presenting data on the sensitivity, specificity, PPV and NPV of Widal test compared to other tests independent of study design and without restriction of publication date. Reports of original studies and review articles written in English language were considered. Studies available only in abstract form, unpublished theses, dissertations and articles with sample size of less than 60 were also excluded ( Figure 1).

Methodological quality assessment
Using culture method as a gold standard, sample size and use of right statistical measurement to assess the diagnostic performance of Widal test were noted as quality of indicators. All assessments were entered into pre-formatted standardized data extraction forms. Studies were assessed for quality and studies fulfilling 75% of quality assessment parameters were included for analysis. High quality studies were: studies that reported outcomes on at least 50 samples, cross-sectional studies and surveillances whose response rate were greater than 80% and used culture as a gold standard.

Data abstraction
The data abstraction was conducted independently by two of the investigators (HMM, KT). The selected studies were reviewed by using pretested and standardized abstraction form to extract data about title; authors, year of publication, country, study design, study site, study base (population-based or hospital-based), sample size, data collection procedure and response rates. When there was a discrepancy in data abstraction, it was resolved through consensus among the team of investigators.

Statistical analysis
Epi-info version 3.5.1 and SPSS version 24 software's were used for data entry and analysis, respectively. The mean, standard deviation, range and median sensitivity, specificity, NPV and PPV of the articles were drawn. The difference was measured using P value ≤ as a significance level.

Summary of articles included in the analysis
A total of 16 articles were included in the systematic review with the oldest publication in the year 1994 and the recent in 2015. The reviewed articles included 50 sample size with the smallest and 1735 samples with the largest. Ten cross sectional and 6 case control studies were reviewed (Table 1).

Sensitivity, specificity, PPV and NPV
Sensitivity is the probability that a truly infected individual will test positive whereas specificity is the probability that a truly uninfected individual will test negative. Positive predictive value (PPV) is the probability that those testing positive by the test are truly infected and Negative predictive value (NPV) is the probability that those testing negative by the test are truly uninfected.
The mean sensitivity Widal test is 73.5 ± 12.6 (95% CI: 60.9-86.1). The probability of a true typhoid patient to be positive by Widal test ranges from 60.9% to 86.1%. Therefore 13.9% to 39.1% of true typhoid patients will be falsely negative by Widal test. The lowest sensitivity of Widal test was 45.2% and the highest is 98%. The mean ability of Widal test to declare Salmonella uninfected febrile patients as negative falls between 55.5% to 95.9%. This indicates that 4.1% to 44.5% of true negatives test falsely positive by this method compared to blood/stool culture methods. The lowest specificity of Widal test was 13.8% and the highest was 98%. The mean PPV of the Widal test is 60% ± 29% (95% CI: 31% to 89%) and the mean NPV of Widal test is 75.2% ± 24.8% (95% CI: 50.4% to 100%) ( Table 2).

Conclusion
The systematic review results show that the reliability of Widal test is comparatively poor. The mean sensitivity, specificity, NPV and PPV of Widal test remains below 80%. The efficiency of Widal test in diagnosing typhoid fever without other confirmatory tests is not of diagnostic value. Therefore, Widal test should not be used as a diagnostic tool to rule out typhoid fever unless supported by invasive clinical pictures and other confirmatory tests.