Contact details
Dr David Moore
Reader in Infectious Diseases and Tropical Medicine
Currently based in Lima, Peru - Contact: 00 511382 3398
Research overview
Over 5000 people die of tuberculosis every single day. This is a curable illness. The major burden of this avoidable mortality and associated morbidity is in the developing world. The WHO DOTS strategy has had some notable, but limited, success in turning the tide of this growing burden, and the spectre of emergent multi-drug resistant disease (MDR-TB) worldwide demands urgent redoubling of control efforts. The development and rigorous evaluation of novel diagnostic tools and innovative strategies of implementation for these tools is the theme of our current research programme in urban Lima, Perú.
MODS
MODS, which was developed in our laboratory at Universidad Peruana Cayetano Heredia in Lima (Caviedes L et al, Rapid, efficient detection and drug susceptibility testing of Mycobacterium tuberculosis in sputum by microscopic observation of broth cultures. J Clin Micro 2000; 38 (3): 1203-8) relies on two long-known features of mycobacterial microbiology: (1) M tuberculosis grows more rapidly in liquid (broth) medium than on solid phase media and (2) characteristic “tangles” of M tuberculosis can be visualized under the microscope long before colonies are visible to the naked eye.
Use of 24-well tissue culture plates and an inverted light microscope, and incorporation of anti-TB drugs at the outset, enables detection of M tuberculosis and direct drug susceptibility testing in a median of 7 days from the time the specimen arrives in the laboratory. This extremely fast, low-cost tool has tremendously exciting potential for future use in high-burden, resource-poor settings – determining the optimal format and strategy of implementation will be key to fulfilling this potential.
TB diagnosis – what we do now.
The vast majority of TB in the world is diagnosed by sputum smear microscopy, a test largely unchanged in over 100 years. This approach has been rolled out in the DOTS strategy because it is cheap, relatively straightforward and detects the most infectious cases. Previously this was indeed a valid approach and it was acceptable to treat patients empirically with first line drugs.
And what we should be doing
However it is increasingly clear that this approach is no longer sufficient, particularly where multidrug resistance (and now XDR) and HIV infection. Smear microscopy misses half of all cases (low sensitivity) and tells us nothing about drug susceptibility. Empiric treatment doesn’t work for MDR TB. Sputum smear microscopy is not deemed as acceptable in the industrialised world and it should not be regarded as an acceptable standard of care in high TB-burden settings where resources are limited but need is greatest. The development of MODS attempts to address this disgraceful inequity by bringing high performance diagnostic testing for TB and MDRTB within reach of national TB control programmes with limited resources.
An operational evaluation of MODS in Lima, Peru
In 2005 we completed a two-year operational evaluation of the field performance of MODS in a study conducted in 15 health centres and two hospitals in Lima. The Wellcome Trust-funded project was undertaken in collaboration with the Peruvian National TB Programme and involved the recruitment of three groups of patients:
1. unselected TB suspects self-presenting or referred to the TB programme for diagnostic testing (most complaining only of prolonged cough)
2. pre-screened “high-risk” TB suspects with either epidemiological risk factors for TB or MDRTB, or with constitutional symptoms (fever, sweats, weight loss)
3. unselected HIV-positive individuals admitted to hospital (for whatever reason)
Sputum samples were cultured in parallel in MODS, automated MBBacT mycobacterial culture and on Löwenstein-Jensen slopes to enable a head-to-head comparison of MODS with tests regarded as reference standards in industrialised and resource-limited settings. Performance in detection of M tuberculosis was evaluated in terms of sensitivity, specificity and detection speed.
The capacity of MODS to rapidly distinguish drug-resistant from drug-sensitive M tuberculosis through concurrent direct drug susceptibility testing (DST) is a major advantage, though direct DST for M tuberculosis is conventionally viewed with some suspicion because of the inability to control the dose of inoculum. In this study we assessed the reliability of that data by comparing MODS results for rifampicin, isoniazid, ethambutol and streptomycin susceptibility testing with the twin reference standards of indirect DST of cultured strains by automated MBBacT and the proportion method.
The principal remarkable findings were published on October 12th 2006 in The New England Journal of Medicine. NEJM 
In brief, M tuberculosis was detected much more quickly by MODS than by MBBacT and LJ (median 7 vs. 13 vs. 26 days) and with much greater sensitivity (98% vs. 89% vs. 84% respectively). Performance in identification of multidrug resistance (resistance to at least isoniazid and rifampicin) was excellent, though reliability in discerning ethambutol and streptomycin resistance was good but not good enough to recommend routine use. Median time to detection of MDR was 7, 22 and 68 days in MODS, MBBacT and LJ/proportions method respectively.
This study and a further analysis of cross-contamination risk in MODS MODS Cross contamination DMID 2006.pdf have enabled us to rationally streamline the methodology and remove redundancy, by reducing the number of control wells required, testing only one concentration of two drugs (previously 2 concentrations of four drugs) and reducing the time a culture needs to be retained from 40 to 21 days..
MODS is thus now a 4 well test for detection of TB and MDR comprising 2 drug-free control wells and one well each for one concentration of rifampicin and one concentration of isoniazid. At less than $2 per test this should be a potentially important step forward for TB and MDRTB diagnosis in resource-limited settings where the burdens of TB and MDRTB are greatest.
Biosafety and MODS
A misconception has arisen about the perceived laboratory biohazard associated with the MODS methodology for TB and MDRTB diagnosis¹. This is partly because MODS utilises liquid media and partly because the full procedure has not been clearly understood.
Contrary to this misconception we firmly believe that MODS is actually considerably safer than all indirect TB drug susceptibility methods.
This is why.
The hazard of positive cultures in liquid media arises from spillage risk and manipulation of cultures teeming with organisms.
Conventional wisdom recognises that liquid TB culture media (such as that used in MODS, MGIT, BACTEC and MBBacT systems) pose a greater biohazard than solid media because of the risk of spillage, aerosolisation and (less importantly) larger mycobacterial loads.
Manipulation of positive liquid cultures is particularly hazardous². Indeed, this essential element of the process for performance of standard indirect (secondary) drug susceptibility testing (DST), involves the handling of mycobacterial loads thousands of times greater than that found in pre-inoculation clinical specimens, thus dwarfing the risks associated with the handling of sputum samples.
MODS utilises direct DST thus involves no culture manipulation
As a methodology utilising direct DST, MODS actually results in considerably less biohazard exposure risk than any method requiring isolate manipulation.
Spillage risk is nullified by handling MODS plates in zip-lock bags
Because inoculated culture plates are sealed within polythene ziplock bags from which they are never removed (microscopic examination is done through the transparent bags), the culture amplification of M tuberculosis and MDR testing effectively occurs within a closed system. The only consequence of a dropped plate is a spoiled culture as any spillage is completely contained.
Biosafety requirements - P3 is neither necessary nor appropriate
Biosafety level 3 standards require, amongst other things, a ventilation system with uni-directional airflow, external exhausting of laboratory air and careful continuous maintenance. The risks associated with ventilation systems which fail are well known and increasing complexity demands increased maintenance expertise and costs which are prohibitive in the resource-limited settings where TB is most prevalent. Safe TB culture requires:
- a well-organised laboratory with a biological safety cabinet (BSC)
- appropriate protective clothing and NIOSH approved respirators used at all times by laboratory workers
The risks associated with TB laboratory work relate primarily to sample preparation, culture manipulation and waste disposal. Culture manipulation is by far the most hazardous of these as highly concentrated aliquots of mycobacteria are exposed to the laboratory air; thus a call for P3 facilities might be justified² when indirect DST is performed.
However this does not apply to MODS for which a well-positioned and properly maintained class II BSC re-circulating exhausted HEPA filtered air into a closed room is more than adequate. This cleans room air and provides a safe environment at lower capital and maintenance cost, more than adequate for the MODS methodology in which the only potential period of risk is in sample preparation for plate inoculation.
- Moore DA, Evans CA, Gilman RH, et al. Microscopic-observation drug-susceptibility assay for the diagnosis of TB. The New England journal of medicine 2006;355(15):1539-50.
- Iseman MD, Heifets LB. Rapid detection of tuberculosis and drug-resistant tuberculosis. The New England journal of medicine 2006;355(15):1606-8.
See also:
Moore DAJ, Gilman RH, Friedland JS.
MODS assay for the diagnosis of TB.
N Engl J Med 2007; 189.
MODS implementation – the next crucial steps
However, the simple availability of a cheap, low-tech methodology is unlikely to be sufficient and our current foci are on further simplifying the test and evaluating a range of different strategies of implementation. This ongoing project in collaboration with the Peruvian TB Programme and the Instituto Nacional de Salud in Lima, generously funded by The Wellcome Trust, will define how best the MODS methodology should be utilised by the healthcare system and will provide insights into how to optimise future roll-out.
UPCH link: http://www.modsperu.org/
Selected publications
Lancet correspondence April 05.
Paediatric string test BMC ID 2006
Traditional medicine and TB diagnostic delay AJTMH 2005
Trop Doc 2005 Diagnostic algorithm absconders
HIV-related tuberculosis in South Africa: clinical features and outcome. Wilkinson D, Moore DAJ. SAMJ 1996; 86 (1): 64-67
Paediatric enema syndrome in a rural African setting. Moore D, Moore N. Ann Trop Paeds 1998; 18 (2): 139-144
Neutrophil adhesion molecules in HIV disease. Moore D, Henderson D, Gazzard BG. Clin Exp Immunol 1998; 114 (1): 73-77
Reversal of abnormalities of neutrophil adhesion molecule expression in HIV infection following protease inhibitor therapy. Moore D, Henderson D, Gotch F, Gazzard BG. AIDS 1998; 12 (15): 2083-4
A retrospective study of neutropenia in HIV disease. Moore DAJ, Sullivan A, Hilstead P, Gazzard BG. Int J STD & AIDS 2000; 11: 8-14
How generalizable are the results of large randomized controlled trials of antiretroviral therapy? Moore DAJ, Goodall RL, Ives NJ, Hooker M, Gazzard BG, Easterbrook PJ. HIV Med 2000; 1: 149-154
Etiology and natural history of neutropenia in HIV disease – a prospective study. Moore DAJ, Benepal T, Portsmouth S, Gill J, Gazzard BG. Clin Infect Dis 2001; 32: 469-475
High rates of tuberculosis in end-stage renal failure: the impact of international migration. Moore DAJ, Lightstone E, Javid B, Friedland JS. Emerg Infect Dis 2002; 8 (1): 77-8
African trypanosomiasis in UK safari-tourists. Moore DAJ, Edwards M, Escombe R, Agranoff D, Bailey JW, SquireSB, Chiodini PL. Emerg Infect Dis 2002; 8 (1): 74-6
Successful treatment of anisakiasis with albendazole. Moore DAJ, Girdwood RW, Chiodini PL. Lancet 2002; 360: 54
Assessing the severity of malaria. Moore DAJ, Jennings RM, Doherty TF, Lockwood DN, Chiodini PL, Wright SG, Whitty CJM. Br Med J 2003; 326:808-9
Gnathostomiasis: an emerging imported disease. Moore DAJ, McCroddan J, Dekumyoy P, Chiodini PL. Emerg Infect Dis 2003; 9 (6): 647-650
Risk factors for malaria in UK travellers. Moore DAJ,Grant AD, Armstrong M, Stumpfle R, Behrens R.Trans Roy Soc Trop Med Hyg 2004; 98: 55-63
