PCR for Diagnosing Tuberculous Fistula in Ano

INTRODUCTION: Fistula in ano is a common surgical problem encountered in the out patient clinic. It is a condition which can be often diagnosed clinically with meticulous examination. It is mostly due to a benign cause which can be treated with surgical therapy therapy. The treatment, although mainly surgical, involves consideration of certain systemic causes and prompt diagnosis and treatment of the same. Tuberculosis is one such ubiquitous cause, especially in developing countries. Various studies have found tuberculosis to be the cause to varying extents in cases of fistula in ano. However, the exact magnitude of the disease burden is not known. Another problem with tuberculosis, in any form, is the difficulty in making a prompt diagnosis in order to start anti-tuberculous therapy. Even though a battery of tests exist, only some tests have stood the test of time and have been considered as gold standard. These tests are microscopy and culture demonstrating presence of M. tuberculosis in the specimen in question. However, even these tests have their shortcomings, a fact which has led to the quest for faster and more accurate tests. One such test is Polymerase Chain Reaction (PCR), a test which allows the amplification of a specific strand of DNA in a sample by enzymatic reaction, such that trace amounts this DNA (which represents the Nucleic acid of the species to be detected) are picked up and amplified, thus confirming the diagnosis. Fistulae caused by M.tuberculosis are difficult to diagnose because of low concentration of the acid fast bacilli in the tissue. Another factor is the overlap in the histopathological appearance between tuberculosis and other infections causing granulomatous inflammation. The present study tries to answer two of the questions regarding tuberculous fistula in ano. It tries to determine the prevalence of tuberculosis as a cause of fistula in ano. It also tries to look at how useful PCR is in diagnosing tuberculous fistula in ano. AIMS & OBJECTIVES: Aims: 1. To evaluate the prevalence of tuberculous fistula in ano in patients attending a tertiary care centre. 2. To determine the sensitivity and the specificity of PCR in the diagnosis of Tuberculous fistula in ano. Objectives: 1. To establish the disease burden of tuberculous fistula in ano among patients seen in the out patient department of a tertiary care centre (Christian Medical College) in Vellore. 2. To establish the role of PCR as a reliable method to diagnose tuberculous fistula in ano. The study aims at evaluating PCR as a tool for quick and reliable diagnosis of Tuberculous fistula in ano where the histopathology of the tract is indeterminate, thus saving the patient a 6-8 week wait before initiation of Anti-Tuberculous Therapy. MATERIALS AND METHODS: Study design – The study was a prospective blinded crossectional study with the evaluation of a diagnostic tool. Patients, clinical information, and clinical specimens. Patients with fistula in ano were drawn from those attending the Out patient department of General surgery Units II and V of the Christian Medical College, Vellore over a period of 20 months (from September 2007 to April 2009). A detailed clinical history, physical examination, baseline laboratory investigations to asses fitness for surgery were conducted for all patients. At the time of the operation, a sample of the curetted fistulous tract was obtained and was cryo preserved at -196 degree C for later use. Bits from the same fistulous tract curettage were sent for AFB smear and culture examination and hematoxylin and eosin histopathological examination (HPE). The HPE reports were followed up subsequently. The AFB culture were followed up for a period of 10 weeks for growth of Mycobacterium tuberculosis. The cryopreserved specimens were subjected to PCR for the presence of IS6110 amplification target. The clinical diagnosis was not known to the laboratory personnel conducting the PCR tests. Sample size: The sample size was calculated by using the prevalence as established by a previous retrospective study done in the Department of General Surgery Unit V which had determined the prevalence of tuberculous fistula in ano as 6.6%. (ref. unpublished data). The formula used for calculation of the sample size was 4pq/d2, where P is the expected prevalence, Q is (1-P), and D is the degree of accuracy expected out of the study. It was decided to do the study with an expected degree of accuracy of +/- 3%, as this would need a sample size which could be completed in the study period. Therefore, using this formula, a sample size of 263 was calculated. Statistical Analysis: The patients whose samples are used in the study were analyzed by way of their distribution of age, sex, number of fistulae found on clinical examination and total number of procedures done for the same complaints. The prevalence of tuberculosis was thus calculated based on the number of patients positive for the gold standard tests divided by the total number of patients examined over the study period. The number of samples which were positive for AFB smear and/or are positive for tuberculosis on histopathological examination were grouped together as positive for the gold standard test. CONCLUSIONS: 1. Tuberculosis is an rare but definite cause of fistula in ano. It should be actively ruled out by testing specimens by histopathological as well as microbiological (Culture) means. 2. Fistula in ano is a chronic disease affecting mostly males. 3. Tuberculous fistula in ano can exist as and entity independent from pulmonary or other systemic involvement of the disease. 4. TBPCR does not appear to be an useful test in diagnosing tuberculous fistula in ano due to a high rate of false positive results. However, in culture positive samples, TBPCR is 100% sensitive in detecting the same