Advances in Tuberculosis Diagnostics

Advances in Tuberculosis Diagnostics

The ability to diagnose drug resistance is improving treatment strategies and outcomes

Feb 27, 2019
Laura M. Bolt, PhD
Jeffrey D. Cirillo, PhD

Jeffrey D. Cirillo, PhD, is a Regents' professor in the Department of Microbial and Molecular Pathogenesis at the Texas A&M College of Medicine. Dr. Cirillo's research interests are in the pathogenesis of bacterial lung infections including tuberculosis, Legionnaires' disease, and other pneumonia agents. He has been a tuberculosis researcher for more than 30 years. Dr. Cirillo leads the Center for Airborne Pathogens Research and Tuberculosis Image Resources and the Small Animal Model Vaccines and Pathogenesis group. He has been awarded over $5 million from the Bill & Melinda Gates Foundation's Global Health Program and over $1 million from NIH, supporting his work on real-time optical imaging and detection of tuberculosis.

Q: HOW IS TUBERCULOSIS (TB) TYPICALLY DIAGNOSED IN INDIVIDUAL PATIENTS? HOW ACCURATE ARE THESE METHODS? 

A: TB is typically diagnosed by acid fast staining, or smear microscopy, and bacterial culture. The other main diagnostic methods are X-ray and CT, which are both not specific. Bacterial culture is the gold standard of diagnosis, but it takes six weeks to complete and can often become contaminated. Culture also requires very good facilities, which don’t exist in the places where the most TB exists. Most cases throughout the world use acid fast staining due to its ease of application. It is the least expensive test and does not require good facilities, but this technique is not very sensitive. Many TB cases that use acid fast staining for diagnosis are missed during early acute infection at 2-4 months, which means that the disease may be transmitted to other people prior to diagnosis. Diagnosis by acid fast staining usually occurs around 5-6 months after symptoms like a cough develop. Due to the delay in diagnosis, more cases occur, and the infection is not diagnosed until symptoms are severe. The infection is then more difficult to treat because more bacteria are present. Treatment requires a patient to take four drugs for 6-9 months, so any increase in difficulty of treatment is a very serious issue and may lead to greater drug resistance. In the case of drug resistance, as many as 60 percent of patients die that have drug-resistant TB strains. 

Q: ARE THERE ANY DIAGNOSTIC METHODS THAT ARE BOTH FAST AND SPECIFIC?  

A: The main specific methods include interferon gamma release assays, like the QuantiFERON Gold test, or nucleotide-based tests, like the GeneXpert test. QuantiFERON is very sensitive, but it loses specificity because individuals that have been previously exposed to TB are positive. GeneXpert is not as sensitive as culture, but it has very high specificity. Like bacterial culture, both the QuantiFERON and GeneXpert tests require very good facilities, making it difficult to diagnose TB where the highest numbers of cases occur. GeneXpert takes only two hours to diagnose but is the most expensive diagnostic test. Overall, there is a great need for a simple, cheap, and fast diagnostic test for TB. Currently, there is no such test.

Q: WHAT ARE SOME KEY ADVANCES IN TB DIAGNOSTICS THAT YOU’VE OBSERVED OVER THE LAST 20 YEARS? WHAT ABOUT OVER THE LAST FIVE YEARS?

A: QuantiFERON and GeneXpert are the two diagnostics that have had the greatest impact over the last twenty years. GeneXpert has probably had the greatest impact up to this point because of its high specificity and a sensitivity similar to or better than acid fast stain. In some places, GeneXpert is even accepted in place of acid fast stain now, which is a major change. Over the past five years, GeneXpert and similar nucleotide-based tests have gotten more sensitive, which is the main change that are we are seeing. Researchers are also developing drug-resistance tests, with the resistance to rifampicin (RIF) test for GeneXpert already available in some places. Being able to diagnose drug resistance in addition to TB is critical and can improve treatment strategies and outcomes.

Q: WHAT ARE THE GEOGRAPHICAL TRENDS IN TB DIAGNOSTICS?

A: There are a number of trends in TB diagnostics, mostly based on cost and infrastructure needs for the tests. GeneXpert was introduced initially in Africa and Asia and is widely used there, but it is also being increasingly used in Europe and the U.S. Culture has been widely used in the U.S. and Europe for some time, but only rarely in Africa and Asia until recently. QuantiFERON is widely used in the U.S., but not in Asia and Africa, where much of the population would test positive. Most of the innovative diagnostics have come out of the U.S. and Europe, but they have been primarily applied in Africa and Asia. This means that the site of development has not greatly impacted where the diagnostics have been applied. Often TB diagnostics have been applied most where the greatest need is, which seems like the best approach.

Q: WHAT ROLE HAS YOUR OWN RESEARCH PLAYED IN ADVANCING TB DIAGNOSTICS?

A: My own research laboratory is developing a series of diagnostic tests based on a new technology called reporter enzyme fluorescence for TB (REFtb). Initially, we plan to complete our sputum-based REFtb test, followed by urine and feces-based tests for the pediatric and HIV+ populations. We are also working in parallel on a drug-susceptibility test (DST) for TB that uses REFtb-DST technology. We believe that these new tests will have a major impact on diagnosis of TB because they are fast, inexpensive, and require no infrastructure. REFtb requires no sample processing and only needs a reagent added at room temperature. It costs less than $2 USD per test and provides results in 10 minutes. It is also a portable test that is hand-held and battery-operated. We have tested this system in over 160 suspected TB patients and the specificity and sensitivity are both greater than 80 percent. We envision this test to be used as a triage test for identification of patients in all settings, even in very remote areas where most cases occur. Patients identified would initiate treatment while they await confirmation with additional tests. More patients could be identified and treated earlier, preventing a great deal of transmission and reducing the overall incidence of TB.  Earlier treatment would also be expected to improve outcomes because of lower bacterial loads at the time of treatment.

Q: HOW DO YOU ENVISION TB DIAGNOSTICS CHANGING IN THE FUTURE?

A: Diagnosis of TB should be very fast. A patient should be diagnosed during their first visit to a clinic, usually after around 2-3 months of having a chronic cough. They should also be prescribed effective drugs for treatment during their initial diagnosis. I would expect confirmation with at least one other diagnostic method to remain necessary because an incorrect diagnosis of TB is a serious issue for the patient. Cost and infrastructure are currently some of the main limitations for TB diagnosis, so if these were removed, GeneXpert would immediately have more widespread use and acid fast stain would no longer be necessary. Our own test, REFtb, is newly developed, so we expect to improve its specificity and sensitivity over the next few years. Since REFtb is a 10-minute test that requires no processing, it could be applied anywhere. In a perfect world, I would envision using only these two tests to diagnose patients, so that all patients could be diagnosed within a maximum of 2 hours for both TB and drug resistance. 


Laura M. Bolt, PhD

Laura M. Bolt, PhD, is a writer, researcher, and university-level educator based in Toronto, Canada. She holds degrees from the University of Cambridge (UK), University of Toronto, and Queen’s University (Canada). Her published and ongoing research projects focus on primate behavioral ecology and communication.