DIAGNOSIS OF TB DR. KONG PO MARN KONG CLINIC FOR CHEST & INTERNAL MEDICINE PATHOGENESIS OF TB INFECTION AND DISEASE (I) PATHOGENESIS OF TB INFECTION AND DISEASE (II) MODES OF DIAGNOSIS • Microbiologic • Smear • Culture • Others ( MODS etc) • Molecular • Nucleic Amplification Assays (NAA) • IFN-γRelease Assays (IGRA) • Line-probe PCR for drug sensitivity testing • Urinary Assays CURRENT PROBLEMS • Active disease • Smears are insensitive • NTM make up 5% of positive smears locally • Cultures take too long ( 12 to 14 days to be positive) • Latent TB Infection (LTBI) • Mantoux test since 1981 • BCG, NTM and boosting can confound issue • Low sensitivity and specificity • Repeat visits NEW MODALITIES IN THE DIAGNOSIS OF ACTIVE TB • NAA • Brochoalveolar lavage with Elispot • Urinary assays ? NUCLEIC AMPLIFICATION TESTS/PCR • Rationale • Isolation issues • Issues with current TB testing • MDR-TB • Standardized and come in kit forms NAA • Amplicor: DNA PCR • Amplified / Enhanced MTD / MTA: rRNA detection • Both FDA approved • In 1999, E-MTD approved for use in both smear positive and negative specimens NAA • Sensitivity is markedly improved cf. smears • MTD: Sensitivity usually range from 83 to 98% in smear + cases. 70 to 81% in smear negative cases. Amplicor slighly less • Specificity is 98 to 99% • Negative predictive value is more important NAA • E-MTD / MTA • Improved sensitivity in smear negative cases • 1999 study. 489 inmates in Texas prison • Overall sensitivity of 95.2% and specificty of 99.1%. • 100% in smear positive cases • 90.2% and 99.1% in smear negative cases • Ontario study (1999) using clinical diagnosis as reference gave nearly 100% specificity and sensitivity for both smear + and sm- cases (Case selection !!) Figure 2. Estimated Predictive Value of Acid-Fast Bacilli Smear and Enhanced Mycobacterium tuberculosis Direct Test by Clinical Suspicion for Tuberculosis Positive predictive value is percentage of patients testing positive who had tuberculosis; negative predictive value, percentage of patients testing negative who did not have tuberculosis. Catanzaro, A. et al. JAMA 2000;283:639-645 Copyright restrictions may apply. NAA FOR SHORTENING ISOLATION • Campos et. Al. AJRCCM 2008;178:300-305 • 493 TB suspects in respiratory isolation • AFB smears vs in house NAA • 46 culture positive cases • NAA detected 40 cases and all smear positive cases • Sens 0.87 and specificity of 1 • Smear sens 0.76 and specificity 0.96 • Average RI tooks 5 days CONCLUSION FOR NAA • Useful in excluding disease in low suspicion patients and ruling it in in those with intermediate and high suspicion. • It is a tool to support diagnosis, not a diagnosis itself. • Potential to decrease isolation use BAL & ELISPOT DIAGNOSIS OF LTBI • LTBI has about 20% chance of breakdown acc to MTX • Potentially infectious when it becomes disease • In use since 1891 • Shortcomings • BCG cross reactivity • False positives and negatives • User variability • Variations in reagents and strength • Poor sensitivity and specificity PROBLEMS WITH THE MTX (WHO HAS LTBI?) • Based on epidemiologic curves • Operator issues • Reagent issues..us!! • Environmental / Locality issues • BCG vaccination(Repeat BCG vaccinations!) • Immunesuppression / anergy • No gold standard • TB disease • Exposure studies GAMMA IFN ASSAYS • Basis • Mantoux test based on weakened M tuberculosis. Cross reacts with BCG • RD1 – region of differentiation • Antigens nearly unique to M tuberculosis • ESAT 6, CFP-10 and TB7.7 • Exceptions: M kansasii, M marinum • Does not cross react with BCG • No gold standard • No track record. Breakdown? GAMMA INTERFERON ASSAYS • Quantiferron, Elispot • Initial data suggests usefulness in detecting LTBI • Advantages • 1 blood draw • No cross reactivity with BCG • No boosting phenomenon ( known) • Disadvantages • Expensive • Advanced equipment • No Gold standard and no track record • NEED TO RISK STRATIFY! Spontaneous reversions QUANTIFERON -GOLD • QFN using ESAT-6 and CFP-10 antigens • ELISA assay • New in tube formulation • QTF Gold TB in tube ELISPOT • More sensitive, specific and convenient • Mononuclear cells from single blood sample stimulated with ESAT-6 • Elispot can detect 1 in 60,000 IFN- secreting cells • Positive in 96% of TB patients ( vs 50% in Quantiferon) and 85% of presumed LTBI • Negative in 26 BCG vaccinated subjects T SPOT TB • PBMC separated and washed • Incubated in antibody ( to IFN) coated wells • Antigens added and Negative ESAT-6 incubated Ctrl • Cells and supernatatnt removed • Antibodies added to highlight the IFN secreted Positive • Machine counted CFP-10 control IGRA SENSITIVITY AND SPECIFICITY • Mori et,al. AJRCCM 2004; 170:59-64 • 216 Japanese nursing students, 152 TB suspects of which 119 were proven TB. • No history of TB exposure • All BCG vaccinated • Using Quantiferon-GOLD • ROC curves to establish cutoff • 64.6% were MTX positive ( specificity of 35.4%) but only 1.9% were ESAT-6 or CFP-10 positive Mori et.al AJRCCM 2004; 170:59-64 • Mori et.al • Overall sensitivity was 89% and specificty 98.1% • Issues • Shows high specificity • Sensitivity in LTBI not established • Expect better sensitivity with LTBI as -IFN levels appear suppressed in active disease cf with LTBI. • Ewer et.al. Lancet 2003; 361:1168-73 • School outbreak in UK • 1128 students screened with Heaf test. Detected 69 TB cases and 254 cases of LTBI • 535 students enrolled in RD1 Elispot study • Detailed contact information obtained • Estimate of OR for Elispot with increased exposure Ewer et.al Lancet 2003; 361:1168-73 Ewer et.al Lancet 2003; 361:1168-73 • Results • Elispot & TST + RR 17.6 • Elispot + but TST - RR 11.7 • Elispot - and TST + RR 2.97 • TST+ related to BCG status . Elispot wasn’t • Ferrara et.al AJRCCM 2005; 172:631-635 • QFT-Gold study in a hospital of 318 patients over 8 months. Tb suspects • 68 indeterminate results • Indeterminate results significantly overrepresented in TST- patients • Indeterminate results more common in patients on immunosuppressive treatment • Indeterminate results may be useful . Ferrara et.al AJRCCM 2005; 172:631-635 -IFN ASSAYS • -IFN Assays appear promising as replacement for TST • Improved sensitivity would include a group previously undetected. ? Immunesuppressed • In immune suppressed, are they less likely to develop Tb and hence TST negative? • Overall, it could save cost if the kits were cheaper • Recent FDA approval to replace TST! -IFN ASSAYS • RD-1 assays showed superior sensitivity and specificity • Unaffected by BCG • Pooled sensitivity for T-spot, QFT-GIT, TST was 90%, 83% and 89%. • Specificity: 88%, 99% and 85% PREDICTING BREAKDOWN Diel et.al. AJRCCM 2011;183:88-95 PREDICTING BREAKDOWN • Diel et.al. AJRCCM 2011;183:88-95 • 954 contacts between 5/05 till 4/08. 1033 completed • QFT-GIT (20.8%) and TST( 63.3% at 5mm, 25.4% at 10 mm) • 903 contacts not given chemoprevention. 19 TB cases • 100% of cases QFT positive. Progression rate of 12.9% • For TST. 89.5% and 3.1% (5mm). 52.6% and 4.8% (10mm) PREDICITNG BREAKDOWN IGRA IN SUBGROUPS • Immune suppressed • Does not appear affected by diabetes • T-spot appears more sensitive for HIV and generally immunesuppressedcases • Children • Use of TNF blockers CONCLUSION • IGRA offer superior sensitivity and specificity for diagnosing LTBI • Show potential in predicting risk of TB disease progression • Convenience • Currently already recommended as replacement for MTX MDR-TB SENSITIVITY TESTING .