Comparison of microscopic and xpert MTB diagnoses of presumptive mycobacteria tuberculosis infection: retrospective analysis of routine diagnosis at Cape Coast Teaching Hospital

Abstract

Introduction: Tuberculosis is a global health problem that causes 1. 4 million deaths every year. It has been estimated that sputum smear-negative diagnosis but culture-positive pulmonary TB diagnosis contribute to 12.6% of pulmonary TB transmission. TB diagnosis by smear microscopy smear has a minimum detection limit (LOD) of 5,000 to 10,000 bacilli per milliliter (CFU/ml) of sputum result in missed cases and false positives. However, GeneXpert technology, with a LOD of 131-250 CFU/ml in sputum samples and its implementation is believe to facilitate early detection TB and drug-resistant TB case. Since 2013, Ghana health Service (GHS) introduce GeneXpert MTB/RIF diagnostic in all regional hospitals in Ghana, however no assessment of performance between microscopy and GeneXpert TB diagnosis cross the health facilities has been reported. The study compared the results of routine diagnoses of TB by microscopy and Xpert MTB from 2016 to 2020 at the Cape Coast Teaching Hospital (CCTH).

Methods: The study compared routine microscopic and GeneXpert TB diagnosis results at the Cape Coast Teaching Hospital (CCTH) from 2016 to 2020 retrospectively. Briefly, sputum specimens were collected into 20 mL sterile screw-capped containers for each case of suspected TB infection and processed within 24 h. The samples were decontaminated using the NALC-NaOH method with the final NaOH concentration of 1%. The supernatants were discarded after the centrifuge and the remaining pellets dissolved in 1-1.5 ml of phosphate buffer saline (PBS) and used for diagnosis. A fixed smears were Ziehl-Neelsen acid-fast stain and observed under microscope and the remainings were used for GeneXpert MTB/RIF diagnosis. The data were analyze using GraphPad Prism.

Results: 50.11% (48.48-51.38%) were females with an odd ratio (95% CI) of 1.004 (0.944-1.069) more likely to report to the TB clinic for suspected TB diagnosis. The smear-positive cases for the first sputum were 6.6% (5.98-7.25%), and the second sputum was 6.07% (5.45-6.73%). The Xpert MTB-RIF diagnosis detected 2.93% (10/341) (1.42-5.33%) in the first and 5.44% (16/294) (3.14-8.69%) in the second smear-negative TB samples. The prevalence of Xpert MTB-RIF across smear positive showed that males had 56.87% (178/313) and 56.15% (137/244) and females had 43.13% (135/313) and 43.85% (107/244) for the first and second sputum. Also, false negative smears were 0.18% (10/5607) for smear 1 and 0.31% (16/5126) for smear 2.

Conclusion: In conclusion, the study highlights the higher sensitivity of the GeneXpert assay compared to traditional smear microscopy for detecting MTB. The GeneXpert assay identified 10 and 16 positive MTB from smear 1 and smear 2 samples which were microscopic negative.

Keywords: Mycobacterium tuberculosis (MTB); Cape Coast Teaching Hospital (CCTH); GeneXpert MTB/RIF; Ghana; TB diagnosis.

Fig. 1

Diagnostic strategy of Mycobacteria tuberculosis (MTB)

Fig. 2

Percentage of GeneXpert MTB/RIF positive cases across smear-classified AFB Microscopic diagnosis. a. The overall GeneXpert TB prevalence among smear negative. b. The overall GeneXpert TB prevalence among scant smear. c. The overall GeneXpert TB prevalence among smear positive cases. MTB- Mycobacteria tuberculosis. RIF-Rifampicin. Negative-no AFB seen. 1 to 9 AFB per 100 fields was considered rare (scanty). 10–99 AFB per 100 fields was considered moderate (positive +). 1–10 AFB per field (check 50 fields) (positive ++). greater than 100 AFB per 100 fields was considered many (positive +++)

Fig. 3

Prevalence of Mycobacteria tuberculosis (MTB) infections by GeneXpert MTB/RIF among males and females with smear-positive and smear-negative cases

Fig. 4

Prevalence of Mycobacteria tuberculosis (MTB) infections by GeneXpert MTB/RIF across age categories