PPD/TST, BCG and

Now IGRAS

How to Translate the TB Alphabet Into Clinical Practice

Rob Smith MD, MPH

Disclosure and Acknowledgement

The speaker has no relationship with companies who manufacture products used in the diagnosis or treatment of TB infection or disease.

The speaker will not discuss unlabeled uses of commercial products

The speaker thanks John Bernardo for use of some powerpoint images

JAMA, July 23/30, 2008-Vol 300, No. 4

BCG - Bacille Calmette Guerin

• Derived from a strain of M. bovis

• Not accepted/recognized in U.S. as

protection against TB

• Not a standardized vaccine

• Efficacy studies range from 0-80%

• Can confound tuberculin skin test

– But consider patient to be TB infected if

TST positive

Control of TB in the US

Early Dx and Rx of persons with active TB

Investigation of contacts of persons with active TB and Rx of contacts with latent TB or active TB

Targeted testing and Rx for LTBI among individuals at high risk for active TB

Reported TB Cases* United States, 1982–2009

Year

No

. o

f C

ase

s

2009: 11,540 cases (3.8/100,000 pop)

11.4% decr. vs 2008*CDC

10,000

12,000

14,000

16,000

18,000

20,000

22,000

24,000

26,000

28,000

1984 1987 1990 1993 1996 1999 2002 2005 2008

Tuberculosis Cases, United

States, 1993-2008

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,0001993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

No

. o

f C

ases

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Pro

po

rtion

Fo

reig

n-B

orn

U.S.-born Foreign-born % Foreign-born

TB rate:

4.2 per 100,000

Trends in TB Cases in Non-US Born Persons,

Massachusetts, 1999-2009

0

50

100

150

200

250

300

99 2000 01 02 03 04 05 06 07 08 09

Pe

rce

nt o

f Ca

se

s

0

10

20

30

40

50

60

70

80

90

Number of Cases Percent of Cases

YEAR

MDPH - ISIS ** 2009: 44 of 181 (24%) Non-US Born MA cases African

**

Time Interval Between Arrival in the US and TB

Diagnosis Among Non-US Born* Cases,

Massachusetts, 2009 (N=181)

1-5 years,

61, 34%

<1 year, 17,

9%

6-10 years,

23, 13%

11-15

years, 26,

14%

>15 years,

54, 30%

*US Born cases include Puerto Rico

MDPH - ISIS

57% in US > 5yr

J. Clin. MicroBiol. Vol. 47, 2009

Latent vs Active TB

Latent: No sx or physical findings suggestive of TB; positive TST or IGRA; normal CXR…negative respiratory smear/culture if done

Active TB: Sx may include fever, cough, sweats. CXR usually abnormal, but not always. Respiratory specimens usually positive.

- Tubercle bacilli - “dormant”

- Usually positive TST (or IGRA)

- No symptoms

- Normal Examination

- Normal chest radiograph

- NOT infectious

- Sputum smears and cultures are

negative

- Not a “case” of TB

Latent TB Infection

ATS/CDC LTBI Recommendations:*

Diagnostic Testing and Treatment

Target testing to TB high risk- Contacts, medical conditions, homeless, prisoners- Recent immigrants, TST-neg w recent travel, underserved

Do NOT test- Low-risk persons- Known TST-pos- Recent (6wk) live virus vaccination (TST or IGRA)

Populations for LTBI therapy- High TB-risk (including over 35), including recent immigrants- High TB-risk pregnant women

TST+ (IGRA+) and risk stratification ≡ ―infected‖

Process:- History, Examination, and CXR for infected individuals- If NO evidence of disease: LTBI- A decision to test is a decision to treat * AJRCCM, April, 2000

Priorities for Treatment of LTBI

in the United States

Lifetime Risk 20%

• most persons with 10mm TST and

either HIV+ or old, healed tuberculosis

Lifetime Risk 10 to 20%

• persons with recent TST conversion

• most persons < 35 y/o with 15mm TST

and on infliximab

• children 5 y/o with 10mm TST

Horsburgh, CR. NEJM 350:2060, 2004

What is the PPD?

100th anniversary of PPD coming up—one of our most enduring tests

PPD=tuberculin Purified Protein Derivative…distributed by WHO; and several manufacturers

Detects exposure to tb by means of eliciting a delayed hypersensitivity response (48 to 72 hrs post injection)

Precipitate proteins of a

culture filtrate of

M. tuberculosis:

CDC

Administering the TST

Create wheal6-10mm diam.

Reading the Skin Test

Read @ 48-72 hours

Must be measured by a professional TRAINED to read TB Skin Tests

Size of the ―bump‖ is measured, recorded in mm

Tuberculin Skin Test

Source:

CDC/DTBE

Source:

Clinica Medica International,

Ciudad Juarez, Mexico

Classification of TST “Positive”

Reactions >5mm induration: HIV , recent contacts of a

person with infectious TB, organ transplant recipients, other immunosuppressed persons (>15mg/d prednisone for one month, use of TNF antagonists)

>10mm induration: recent immigrants (5yr) from high prevalence countries, injection drug users, residents/employees of high risk congregate settings (ie prisons, hospitals/NH, homeless shelters); X ray evidence of prior healed TB;particular clinical conditions (ir DM, renal failure, silicosis etc)

• >15mm induration:persons w no known risk factors

Forgotten TB:The Boosted TST Response

Principle:

• Immune memory of TB infection fades over time in absence of re-stimulation TST response depends on intact immune memory

Implication:

• TST performed long after TB infection or specific re-stimulation (i.e. a recent TST) can be negative – but the test itself can stimulate immune memory – THEN …

• A subsequent positive TST response may reflect a re-constituted immune memory, not a new, interim infection (e.g., sequential testing HCW)

Solution: 2-Step Testing

• If person has not had a TST in > 1 yr and initial test is negative, repeat TST after 1-3 weeks

• If repeat TST is negative: not infected

• If repeat TST is positive: infected (not a converter)

BCG

80% of world’s population have been vaccinated

Unlikely to cause positive PPD after 10 yrs if no repeat exposure to TB

No difference in prevalence of TST rxns >10mm in young adults vaccinated w BCG as infants and those never vaccinated (Quebec study)

There is a difference in prevalence if BCG given after infancy

BCG may cause positive PPD reactions with a ―boosting‖—ie repeat testing in 1-3 wks

What’s an IGRA anyway?

A product of basic science research: genomic region in M tb not present in BCG strains and most NTMs

Two well studied antigens: ESAT-6, CFP10

M tb elicits a gamma interferon response from particular T cells

This test measures this response to samples obtained from whole blood

Interferon-Gamma Release

Assays (IGRA)

in vitro assays for cell-mediated immunity to M. tuberculosis ~ specific protein antigens

• Utilize live cells in whole blood

• Measure release of IFN-γ by circulating T lymphocytes following stimulation with TB proteins

versus TST, which is …

• in vivo (into the skin) assay for cell-mediated immunity to protein filtrate antigens (multiple, some not specific to MTb) from TB culture (PPD)

• Measure cell infiltration/effects into site where

Species specificity of ESAT-6 & CFP-10

Environmental

strains

Antigens

ESAT CFP

M abcessus - -M avium - -M branderi - -M celatum - -M chelonae - -M fortuitum - -M gordonii - -M intracellulare - -M kansasii + +M malmoense - -M marinum + +M oenavense - -M scrofulaceum - -M smegmatis - -M szulgai + +M terrae - -M vaccae - -M xenopi - -

Tuberculosis

complex

Antigens

ESAT CFP

M tuberculosis + +

M africanum + +

M bovis + +

BCG substrain

gothenburg - -

moreau - -

tice - -

tokyo - -

danish - -

glaxo - -

montreal - -

pasteur - -

Interferon-Gamma Release Assays

(IGRA)

QuantiFERON-TB® approved by FDA in 2001 as ―… an aid to the diagnosis of TB infection.‖

QFT-Gold® test US FDA approved in 2005

QFT-Gold in-Tube® test approved 2007

T Spot-TB® test approved 2008

Revised Guidelines, June, 2010 • CDC (US): http://www.cdc.gov/mmwr/PDF/rr/rr5905.pdf

(MMWR, 6/25/10)

• CTC (Canada): http://www.phac-aspc.gc.ca/publicat/ccdr-rmtc/10pdf/36-acs-5.pdf

Advantages of IGRAs

Single patient visit

IGRA draw does not ―boost‖ subsequent IGRA tests

• But TST may boost subsequent IGRA

Less likely positive in BCG-vaccinated: Specificity

―Objective‖ read-out

Results available in 1-2days

Cost benefits (??)

Culture- and ethnic-naïve

Interpreting IGRA Results

Negative: Same interpretation as negative TST

Positive: Same interpretation as positive TST• Medical evaluation and chest x-ray are

still needed to exclude TB disease and confirm LTBI

Indeterminate: Test failure• Repeat test

Borderline: (T-Spot® only) • Repeat test

CDC: MMWR, 6/25/10

CDC: Whom to IGRA Test?

As with TST, DO NOT test low risk persons

In place of (but not in addition to*) TST in all situations in which CDC recommends TST as an aid in diagnosing M. tuberculosis infection• Includes:

Contact investigations,

Testing during pregnancy,

Screening of healthcare workers and others undergoing serial evaluation for M. tuberculosis infection

• IGRA preferred in: Persons who have received BCG and

Persons who historically have poor rates of return for TST reading

• Except: Children (<5y/o; TST preferred)

Fact Sheet, IGRAs, 2010

* some caveats: e.g., to convince someone that a pos TST is a “true” pos

CDC: What are the Steps in

Administering an IGRA?

Confirm arrangements for testing in a qualified laboratory and

Arrange for delivery of the blood sample to the laboratory in the time the laboratory specifies to ensure testing of samples with viable blood cells.

Draw a blood sample from the patient according to the test manufacturer’s instructions.

Schedule a follow-up appointment … (to review results, etc.)

Fact Sheet, IGRAs, 2010

Limitations of IGRAs:The Challenge

Limited data in many groups, such as those with impaired immune function and children (<5 y/o)

High indeterminate results, poor reproducibility shown in some studies

No good definition of conversion • Currently: negative → positive ≡ conversion

The ability of IGRAs to predict risk of progression to TB disease has not been determined • May be different than in those with a positive

TST

As with TST, IGRAs may be useful as a diagnostic aid within certain clinical

Negative/Positive Predictive Value

of IGRA for Developing Active TB City of Hamburg Germany: N=1414 TB

contacts of smear positive cases; 903 refused chemoprophylaxis

Positive QFT =20% (147); 17 progressed to active TB (12%)

Positive TST= 63% (555) at 5mm; 25% (207) at 10mm…progression rate of 3% and 5%

Neg PV QFT=100%; none of the QFT negative, TST + subjects progressed.

Am J Resp Crit Care Med 2011;183: 88-95.

IGRAs in Persons with HIV

Pts with pulmonary TB/HIV (Tanzania): sensitivity 65% (Decreases with lower CD4 count)…PLoS ONE 2009;4:e4220.

Pts w active TB/HIV (India): QFT-G sensitivity 65%; indeterminate 17%; negative 18%; TST sensitivity 30%.

PLoS ONE 2009; 4: e5718

The future?

Combination biomarker tests to increase sensitivity in immuno-suppressed patients

Combination markers to predict risk of disease activation (see JID 2010; 202: 1685)