2 Recommendations

Antibody detection for TB diagnosis: End of the story?

By Tobias Broger | 14 Mar, 2017

TB serodiagnostic tests have been found to be inaccurate and unreliable, leading the WHO to issue a strong recommendation against the use of serological tests for the diagnosis of active TB (http://apps.who.int/iris/bitstream/10665/44652/1/9789241502054_eng.pdf).
In the 2011 policy statement, the WHO encouraged further research in this area, specifically prospective, blinded studies investigating the performance of serodiagnostics for representative populations with presumptive TB.
 
In our recently published study (https://doi.org/10.1093/cid/cix023), we collected blood samples from 755 adults with presumptive pulmonary TB and measured IgG antibody responses to >100 antigens. Using WHO TPP criteria for a TB detection and triage test, we evaluated single antigen performance as well as models of all possible 3-antigen combinations and models with >3 antigens.
 
Results:
TB detection test results: 35% sensitivity at 90% specificity (TPP minimal target: 65% sensitivity at 98% specificity)
TB triage test results: 34% specificity at 85% sensitivity (TPP minimal target: 70% specificity at 90% specificity)
 
These results suggest that a conventional antigen-based IgG detection test is unlikely to meet TPP requirements in symptomatic patients. This is in concordance with several systematic reviews and meta-analyses.
 
Discussion Questions:
(a) Is this the end of the story, or is there still a role for antibody detection for the detection of active TB?
(b) Is it worth considering antibody detection in combination with detection of other host markers (e.g. cytokines)?
(c) What is the relevance of IgG properties such as FcR-functional profiles (see https://www.ncbi.nlm.nih.gov/pubmed/27667685)?
(d) Could antibody detection be relevant for other target conditions (e.g. progression from latent to active TB)?
(e) Is anyone still using commercial tests based on antibody detection in practice and what is the rational?

Replies

 

Madhukar Pai Moderator Replied at 10:42 AM, 14 Mar 2017

Thanks for this update, Tobi. I think all of us would like a rapid serology test for TB, but much more basic work is necessary to get there.

To answer your last question, China is one country that continues to rely on serological TB tests, despite the negative WHO recommendations. Please see this attached market analysis study from China- in the non-CDC sector, i.e. hospital sector, serology is very commonly used, and there are several Chinese diagnostic companies that make commercial antibody tests.

URL http://www.ingentaconnect.com/content/iuatld/ijtld/2016/00000020/00000003/art...

Attached resource:

Prof Sarman Singh Replied at 11:42 AM, 14 Mar 2017

It is true that in 2010 when all the kits marketed at that time were found
to have suboptimal to dismal sensitivity or specificity or both. The
fundamental reason was that the marketing agencies/vendors did not do
extensive evaluation studies, before dumping these kits in the high TB
burden countries like India. Also the use of these kits was uncontrolled
and highly misused. Nevertheless, no recommendation can be final and in the
dynamic world, new research and innovations should be encouraged, but
watched carefully. Even though very critical to the kits marketed before
2012 in India (doi: 10.4103/0971-5916.90980), we have been of the view that
serological RDTs could be the best triage test. Recently we have identified
5 novel antigens from a clinical isolate, which turned MDR from pan
susceptible form while the patient was on ATT. These antigens have shown
95-98.5% sensitivity and more than 96-100% specificity. The

antigens performed excellent in HIV-TB coinfection. The findings are being
published in Scientific Report next week ( DOI: 10.1038/srep44121). These
findings were based on ELISA and dotblot assays. Now we were eagerly
looking forward to find a commercial partner who can develop rapid
diagnostic test kits using the se antigens.

Prof. Sarman Singh, New Delhi, India

--
============================================
Prof. Sarman Singh, MD, FAMS, FRSC, FRSTMH
Head, Division of Clinical Microbiology & Molecular Medicine,
All India Institute of Medical Sciences, New Delhi-110029,
Phone: +91-11-2658-8484, 2659-4977, Fax: +91-11-26588641, 26588663
President: IAMM-Delhi (2015-2016)
President: Association for Better Health and Society (ABHAS)
President, SIIP (2014-2016)
Editor, Journal of Laboratory Physicians
Academic Editor, Medicine (LWW), Tuberculosis Research & Treatment
(Hindawi)
LinkedIn:
https://www.linkedin.com/in/dr-sarman-singh-1175481b?trk=nav_responsive_tab_p...

Google Scholar:
https://scholar.google.co.in/citations?hl=en&user=LJNBta4AAAAJ

Marc Tebruegge Replied at 11:11 AM, 2 Apr 2017

Dear Tobias,

Well done for completing that study and getting those data published. This undoubtedly represents a huge amount of work, which has clearly evolved over several years.

Despite primarily being a ‘T cell’ person, with most of my research work focused of TB-specific T cell responses, I do not believe that this signals ‘the end of the antibody story’.

There is no doubt that current commercial antibody-based tests have insufficient performance characteristics to be used for clinical care. However, several papers have shown very promising results, albeit most of these were basic science / mechanistic, rather than focused on development or evaluation of a diagnostic test.

I believe there are a number of potential limitations related to your study, which complicate the interpretation of the study findings:

a) As cases classified as non-TB disease (NTBD) did not undergo TST or IGRA – the presence of LTBI has not been ruled out in those individuals. Previous data suggest that a substantial number of individuals with LTBI do have TB antibodies.

b) A significant proportion of cases classified as NTBD may have had low-grade / paucibacillary TB infection (with negative microbiological results) and had ‘self-cure’. There are clearly data from the pre-antibiotic era showing that some patients with active TB are able to ‘cure’ themselves without anti-TB treatment. The key problem in my mind is that no alternative diagnosis was established in the NTBD group (e.g. culture-proven bacterial pneumonia).

c) Given the points made in a) and b) I think there is a considerable potential that data contamination has occurred in that study, which ultimately resulted in very poor performance characteristics for the final panel.

d) Another significant issue with the study is the transfer between different platforms, which introduces further uncertainties. In brief, antigens were selected based on the results of a proteome microarray done in a previous study. These antigens were then expressed (primarily in E coli.) and purified, while other antigens used in the study were native antigen preparations. The antigens were then used in 1. Luminex-based assays, and 2. the MBio immunoassay. Undoubtedly there is the potential for technical issues along the way: a) the antigens expressed in E. coli could have a different structure or conformation than the antigens used in the proteome microarray, b) the epitope orientation of antigens used in the Luminex and MBio arrays may have been suboptimal, c) the MBio arrays themselves may be insufficiently sensitive/robust. In other words, while TB-specific antibodies may have been present in the samples of the active TB cases, they may have not been detected due to technical issues.

Consequently, I believe the study findings have to be interpreted with caution, and are insufficient evidence to conclude that antibody-based TB tests are not feasible.

I am part of a private/public consortium (the MIMIC consortium) funded by the UK Technology Strategy Board / Innovate UK, which is working on a rapid, multiplexed antibody-based test for TB, which involves investigators at University College London, the University of Edinburgh, the University of Glasgow and the University of Southampton, as well as three industry partners. So far we have only screened 31 antigens, and already achieve far greater sensitivity/specificity than reported by your study. Over the next months further antigens will be screened before the final selection is made. Clearly we have a long way to go before arriving at a test that can be used in a clinical setting, but the data are very encouraging.

Also, you may be aware of an abstract related to a study Mahavir Singh and Gerhard Walzl’s group were involved in, which was presented at an EDCTP meeting a few months ago (http://gh.bmj.com/content/bmjgh/2/Suppl_2/A36.1.full.pdf). Of course this was a rather small study (n=63), but their data suggest that sensitivities/specificities in excess of 90% can be achieved with a fairly small number of antigens.

Importantly, overall antibody-based tests have the potential to be cheaper, more robust and more rapid than T cell-based assays or microarray-based tests, and therefore a better option in resource-poor settings, so I believe it would be unwise to give up on this diagnostic approach just yet.

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