Use of polymerase chain reaction for primary diagnosis of pulmonary tuberculosis in the clinical laboratory

Spectrum of Pulmonary Infections in Renal Transplant Recipients in the Tropics: A Single Center Study

BACKGROUND

Pulmonary infections have been implicated as the most common cause of infection related mortality in renal transplant recipients. An appropriate empirical treatment of post transplant pulmonary infections requires knowledge of the spectrum of the microorganisms involved in causing these infections. Besides this knowledge, an aggressive diagnostic approach including the use of invasive tests is often essential to make an early diagnosis for instituting timely and appropriate therapy. We carried out a prospective cohort study to analyze the spectrum of pulmonary infections in these patients and study the utility of bronchoalveolar lavage (BAL) in the diagnosis of the same.

METHODS

From September 2001 to December 2002, 428 patients were under follow up with the department. In all, 40 renal transplant recipients reported with 44 episodes of pulmonary infection during this study period. All patients underwent detailed and appropriate investigations including specific laboratory tests, sputum analysis, X-ray chest, CT and BAL. The spectrum of the causative organisms and the utility of BAL as compared to the other methods of diagnosis were studied and compared.

RESULTS

Out of the 44 episodes of pulmonary infection evaluated, single causative organism could be found in only 24 (54.5%) episodes and multiple etiologies were found in 15 (34.1%) episodes. No definitive cause could be found in 5 episodes. Out of 57 organisms isolated in the 44 episodes, 20 (45.4 %) were bacteria, 16 (36.3 %) each were M. tuberculosis and fungus, 3 were CMV infection and 2 were nocardia. BAL gave a diagnostic yield of 75.8% (25 out of 33 cases). Nine of forty patients died (mortality rate 22.5%) of which 6 deaths could be attributed directly to pulmonary infection. Out of these 9 patients who died, cause of pulmonary infection was bacterial in 5, fungal in 2 and CMV disease in 1. In one patient, organism could not be isolated.

CONCLUSIONS

Our study has shown that more than 1/3rd of pulmonary infections in renal transplant recipients can be attributed to multiple organisms. Bacterial infections were the commonest cause of post transplant pulmonary infection. Tuberculosis is common cause of pulmonary infection in these patients in our set up. Because of its high diagnostic yield, BAL should be considered in all patients with suspected pulmonary infections in the post transplant period.

Assessment by meta-analysis of PCR for diagnosis of smear-negative pulmonary tuberculosis

We conducted a meta-analysis to assess the performance of PCR for the diagnosis of smear-negative pulmonary tuberculosis (SPT) and to identify factors that account for differences in the diagnostic accuracy of different studies. Studies published before February 2002 were included if sensitivity and specificity of PCR in smear-negative respiratory or gastric-aspirate specimens could be calculated. Analysis was conducted by using summary receiver operating characteristics models. Sensitivity and specificity ranged from 9 to 100% and from 25 to 100%, respectively. Fewer than 40% of the 50 studies reported results by number of patients, reported clinical characteristics of patients, or used as a reference standard combined culture and clinical criteria. Studies that included bronchial specimens showed higher accuracy than studies that evaluated only sputum specimens or included gastric aspirates. Studies that did not report that tests were applied blindly showed higher accuracy than those reporting blind testing. Increased sensitivity due to the use of DNA purification methods was associated with decreased specificity. Studies published after 1995, using Amplicor or dUTP-UNG, were associated with an increase in specificity at the expense of lower sensitivity. We concluded that PCR is not consistently accurate enough to be routinely recommended for the diagnosis of SPT. However, PCR of bronchial specimens could be useful in highly suspicious SPT cases. Studies not reporting blind testing are likely to overestimate accuracy of PCR. Future evaluation of PCR accuracy should be conducted by patient and type of respiratory specimen, blindly, by using a reference standard that combines culture and clinical criteria and addresses the issue of how patient characteristics affect PCR accuracy.

Potential use of Amplicor PCR kit in diagnosing pulmonary tuberculosis from gastric aspirate

Polymerase chain reaction (PCR) detection of mycobacteria from gastric aspirate for the diagnosis of tuberculosis is not fully evaluated up to now. A total of 116 gastric aspirate specimens were collected from patients with suspected pulmonary tuberculosis. The breakdown of diagnosis was 67 pulmonary tuberculosis, 16 nontuberculous mycobacterial infection, 5 extra pulmonary tuberculosis, and 28 other lung diseases. The conventional methods were shown to have a sensitivity of 47.8% and a specificity of 79.6%; on the other hand, Amplicor had 34.9% and 97.0%, respectively. The Amplicor provided a more rapid and specific method for diagnosing tuberculosis and was more useful than the conventional.

Diagnostic value of the Amplicor PCR assay for initial diagnosis and assessment of treatment response for pulmonary tuberculosis

We evaluated the Amplicor PCR assay as an initial diagnostic tool on the basis of clinical diagnosis, and assessed this assay as a follow-up test for patients with pulmonary tuberculosis during chemotherapy. Of the 208 specimens from 155 patients who were bacteriologically and/or clinically diagnosed with active tuberculosis before chemotherapy, 144 were Amplicor PCR-positive (sensitivity, 69.2%), which was equal to the results of culturing. Among 89 specimens which showed positive results by smear and culturing, the Amplicor PCR assay detected 87 (97.8%), whereas among 55 specimens which showed smear-negative but culture-positive results, the Amplicor PCR assay detected 46 (83.6 %)(P= 0.003). No false positive results were found in the two systems (specificity, 100%, 120/120). The Amplicor PCR assay was also evaluated as a follow-up test using 926 specimens from 207 patients receiving active tuberculosis chemotherapy. Among 433 specimens which showed Amplicor-PCR positive, 222 (51.3%) were culture-negative. On the other hand, among 233 culture-positive specimens, only 12 (5.2%) were Amplicor PCR-negative. Therefore, this assay is useful for the rapid diagnosis of tuberculosis. The duration of Amplicor PCR-positive after culture-negative conversion was significantly associated with the presence of cavitary lesion, smear-positive specimens before treatment, and smear-positive specimens with negative cultures during chemotherapy.