Davies_edit.qxp 4/2/09 11:47 am Page 75
Bacterial Infections
Latest Developments in the Diagnosis of Tuberculosis
a report by
Peter DO Davies,
1
Biswajit Chakrabarti
2
and Paul Albert
2
1. Director, Tuberculosis Research and Resources Unit, Cardiothoracic Centre, Liverpool;
2. Specialist Registrar, Respiratory Medicine, Clinical Sciences Centre, University Hospital Aintree
From the point of view of a clinician with over 30 years of experience in the 16S ribosomal RNA (rRNA). Amplicon detection is by hybridisation
the diagnosis and management of patients with tuberculosis (TB), the to a probe resulting in chemiluminescence, which is then detected by
advances that have been made in the last 15 years in diagnostic testing luminometer. Using the tube method, the process takes three hours.
for TB have been most welcome. It has been heartening to see new
monies being invested in the latest technology as an aid to the diagnosis In contrast, the Roche Amplicor test targets the 16S rRNA gene
of TB to help us deliver a better service to our patients. through the conventional polymerase chain reaction (PCR) technique.
Detection is through the enzyme-linked immunosorbent assay (ELISA),
However, I also feel a sense of disappointment. Even with the newest, and takes about six hours to complete. In-house, cheaper PCR tests
most expensive techniques, TB remains difficult to identify in many cases. have been developed using different elements of the bacterial DNA as
The easy-to-diagnose patient is still easy to diagnose, and the difficult-to- targets. Studies have suggested that optimal yields are obtained by
diagnose patient remains difficult to diagnose. We still rely on empirical applying PCR-based tests to clinical specimens in broth-based cultures
treatment in a high proportion of our cases. Data from the UK Health to rapidly identify mycobacterium.
2
Although of value for pulmonary
Protection Agency (HPA) show that one-third of our pulmonary cases and TB, where bacteria are usually plentiful, these PCR-based tests have
nearly half of our extra-pulmonary cases are culture-negative.
1
not yet been proved to be reliable in paucibacilliary diseases, such as
tuberculous meningitis.
3
The skills of the clinician are still necessary in eliciting the patient’s
medical history and performing an examination in the diagnosis of TB. In practice, the use of PCR-based testing probably alters management
Above all, clinical suspicion is paramount because if TB is not of the disease only in the event of a smear-positive patient with
considered, the necessary tests to confirm the presence of disease will pulmonary disease having a negative M. tuberculosis PCR test. In this
not be requested. situation, it is likely that the patient has an environmental organism
and not M. tuberculosis, so treatment and contact-tracing activities
The essential tools to diagnose Mycobacterium tuberculosis are the Ziehl- can be modified accordingly.
4
In view of the fact that we treated one-
Neelsen smear or, more commonly, in the resource-rich setting phenol third of our pulmonary patients and nearly half of our extra-pulmonary
auramine and solid Lowenstein-Jensen (LJ) culture. The recently published patients for TB without a positive culture, and bearing in mind that
UK guidelines for the management of TB emphasise the use of liquid PCR has proved to be less sensitive than culture, we are unlikely to be
culture media to speed up the diagnostic process. Early systems utilised more influenced by a negative PCR to stop treatment than we are by
radiometric detection, while modern systems use fluorometric or a negative conventional culture.
colorimetric detection. Optimal recovery is achieved through a
combination of rapid automated liquid culture systems and solid LJ The increasing prevalence of drug-resistant (DR) and multidrug resistant
slopes. Despite advances, three consecutive sputa samples are still TB (MDRTB) is a threat to our ability to control the disease. Traditional
required for the diagnosis of tuberculosis. methods of susceptibility testing can take several weeks; in contrast,
PCR techniques can provide 95% accuracy in rifampicin gene resistance
Against this ‘traditional’ background for the laboratory-based diagnostic within a matter of hours in a sputum-positive individual. For other
tests, the new molecular techniques have proved to be both encouraging drugs, such as isoniazid and ethambutol, rapid testing for resistance is
and disappointing for the clinician: encouraging because they can speed not as reliable due to the complexity of the encoding resistance
up the diagnosis or exclusion of TB in many cases, but disappointing
because they have not improved our overall confirmation rates of disease.
Peter DO Davies is Director of the Tuberculosis Research and
Resources Unit of the Cardiothoracic Centre in Liverpool,
Nucleic acid amplification tests (NAATs) allow rapid and specific diagnosis
which he established in 1990. He has conducted research
of M. tuberculosis infection from a range of clinical samples; however,
into many epidemiological aspects of tuberculosis in
Liverpool and other parts of the world, particularly Hong
commercial validation of NAATs has been given for respiratory samples
Kong and India. In 2004, he was appointed Honorary
only. Although more expensive and less sensitive than culture, the speed Professor to Liverpool University, and he is Secretary and
of these tests and their increased sensitivity and specificity over smear
founder of the tuberculosis charity TB Alert. Professor Davies
is the author of nearly 100 papers published in peer-
microscopy make them popular.
reviewed journals, 30 book chapters, more than 100 letters and commissioned articles and
over 150 abstracts of presentations at scientific societies.
A number of these tests are now approved. The Gen-Probe MTS
E: peter.davies@ctc.nhs.uk
system utilises transcription-mediated amplification, which amplifies
© TOUCH BRIEFINGS 2008 75
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76  |  Page 77  |  Page 78  |  Page 79  |  Page 80  |  Page 81  |  Page 82  |  Page 83  |  Page 84  |  Page 85  |  Page 86  |  Page 87  |  Page 88  |  Page 89  |  Page 90  |  Page 91  |  Page 92  |  Page 93  |  Page 94  |  Page 95  |  Page 96  |  Page 97  |  Page 98  |  Page 99  |  Page 100  |  Page 101  |  Page 102  |  Page 103  |  Page 104  |  Page 105  |  Page 106  |  Page 107  |  Page 108  |  Page 109  |  Page 110  |  Page 111  |  Page 112  |  Page 113  |  Page 114  |  Page 115  |  Page 116