Koch to Quantiferon: An Overview of Tuberculosis testing in the United States

Jamie Ritchey, MPHApril 28, 2008

EPID 755: Emerging Infectious Diseases

Overview• Tuberculosis: Basics and brief History• Screening test validity: Specificity, Sensitivity,

Positive Predictive value, Negative predictive value and disease prevalence

• Tuberculin testBackground, specificity, sensitivity, PPV, NPV, strengths and weaknesses

• Quantiferon-TB in a tube testBackground, specificity, sensitivity, PPV, NPV, strengths and weaknesses

• Public Health Recommendations for Tuberculosis screening in the United States

TB Basics Tuberculosis (TB) in humans is caused by a mycobacterium

tubercle bacillus TB commonly infects the lungs (pulmonary TB), but can also

infect the skin, bones, joints, central nervous, genitourinary, circulatory and lymphatic system(s)

Active pulmonary TB include chest pain, bloody sputum, fever, chills, weight loss, pallor, and fatigue

Latent disease is more common One of the most deadly diseases globally - nearly 2 million

people still dying world wide -death is largely preventable TB has been considered an Emerging Infectious Disease

not due to crossing of a species, but due to its resistance to several drug therapies further complicating disease management and a rapid increase in incidence in many populations

CDC. Questions and Answers About TB, 2007. http://www.cdc.gov/tb/faqs/qa_introduction.htm#Intro1

History of TB up to Tuberculin test• Rothschild et al. reported the oldest Mycobacterium tuberculosis DNA extracted from a bison

over 18,000 years ago (Rothchild, 2001).

• Remains of both pre-humans and mummies have shown TB infection in bones dated from 4000- 2400 B.C. (National Tuberculosis Center, 1996)

• Phthisis a Greek term for TB was also described by Hippocrates in 460 B.C. (National Tuberculosis Center, 1996)

• There is evidence of TB in the Paracas-Caverna culture from 750 BC- 100 AD (South America Pre-historic findings, 2008)

• Biblical references to TB as well (Daniel, 1999)

• In the seventeenth century, Italian medical texts began to describe the pathology and infectious nature of the disease (National Tuberculosis Center, 1996)

• Brehmer, a botany student, retreated to the Himalayans after becoming infected with TB and attributed the fresh air to his recovery concept of Sanatoria was developed (National Tuberculosis Center, 1996)

• Robert Koch discovered tubercle bacillus the causative agent of TB based on his famous postulates and published his classic paper in 1882

• Koch believed tuberculin would cure TB, he discovered this later became the tuberculin skin test used for diagnosis of TB exposure, which is still commonly used today about 118 years later

Basics of Screening Test ValidityScreening

Test result

“True” “False” Predictive value

Positive test True positive

2

False positive

Type I error

18

PPV

2 / 2+18= 10%

Negative Test False negative Type II error

1

True negative

182

NPV

182 / 183= 99.5%

Total Sensitivity

2/3 = 66.7%

Specificity

182/200= 91%

203

• Sensitivity: the ability of the test to produce a positive result in patients that have TB

• Specificity: the ability of the test to produce a negative result in patients that do not have TB

• Positive predictive value (PPV): if the patient has a positive tuberculin test, the probability that the patient actually has TB

• Negative predictive value (NPV): if the patient has a negative tuberculin test, what is the probability that the patient does not have TB

Gordis, 2000; Szklo and Nieto, 2007

Basics of Screening Test Validity

Measure Test 1 Test 2 Test 3 Test 4

Prevalence 50% 20% 20% 20%

Sensitivity 50% 50% 90% 50%

Specificity 50% 50% 50% 90%

PPV 50% 20% 31% 56%

• Here, we can see the relationship between the prevalence of disease, The sensitivity, specificity, and positive predictive value.

• As the prevalence decreases, we can see that increasing the specificity Of a test has more of an impact on PPV compared to sensitivity

• Note that the prevalence estimate for TB in 1971-72 was estimated to be 21.5% (95% CI 17.9-25.1) and may have been subject to sampling variability which would impact the PPV (Engel and Roberts, 1977).

Tuberculin skin testBackground

• OT “Old Tuberculin” developed by Koch 1890’s

• PPD-S developed in 1939 (Seibert) the International Standard for Purified Protein Derivative of Mammalian Tuberculin by the WHO Expert Committee on Biological Standardization

• TB skin test is administered by 0.1 ml PPD containing 5 TU injected into the arm to test for exposure to TB and/or latent TB infection

• TB tests should be analyzed on the second or third day

• Cut-point of 10mm or larger is generally used as indicative of TB infection, but is subjective (sensitivity)

Tuberculin skin testSpecificity

• Historical issue: Pulmonary calcifications, true negatives classified as false negatives

• Environmental Mycobacteria cross-reactions increases false positives

• Baccille Calmette-Guerin (BGG) vaccine increases false positives

Tuberculin skin testSpecificity: Historic Misclassification due to

Pulmonary calcificationsTest Result True False Predictive Value

Positive True positive

2

False positive

18

PPV = 10%

Negative False negative

1

111 181

True negative

182180

2

NPV

182/183= 99.5%

2 / 182= 1%

Total Sensitivity

2/3= 66.7%

2 / 182= 1%

Specificity

182 / 200= 91%

2/20= 10%***Lowers the NPV, specificity, and sensitivity

Tuberculin skin test Specificity: False positive test results

Test Result True False Predictive Value

Positive True positive

2

False positive

18

180

198

PPV = 10%

2 / 200= 1%

Negative False negative

1

True negative

182

180

2

NPV

182/183= 99.5%

2 / 3= 66.7%

Total Sensitivity

2/3= 66.7%

Specificity

182 / 200= 91%

2 / 200= 1%

***Lowers the NPV, specificity, and sensitivity

Tuberculin skin test Sensitivity

• Cut-off point of test impacts sensitivity >10mm is typically used

• Rose et al. calculated sensitivity as 100% at a 2mm cut-off, yet 59% at a 16mm • HIV infection, immunosuppressive chemotherapy, malnutrition,improper administration of the test and younger patient age

Tuberculin skin test Sensitivity: inability to mount an immune response (HIV +, malnutrition, chemo)

Test Result True False Predictive Value

Positive True positive

2

1

False positive

18

PPV

2/18= 10%

1/18= 5.6%

Negative False negative

1

1

2

True negative

182

NPV

182/183= 99.5%

182/184= 98.9%

Total Sensitivity

2/3= 66.7%

1/3= 33.3%

Specificity

182 / 200= 91%

**False negative results cause a drop in sensitivity, PPV and NPV

Tuberculin skin test Cost

• Test itself is low cost, yet manpower and false positive cases are expensive

• Public health departments TB programs costs for health care workers ranged from $92,886 - $291,248 while hospital programs ranged from $66,564-$332,728 (Lambert, 2003)

• Per health care worker, TST programs in health departments ranged in cost from $176-$264 and $41-$362 at hospitals (Lambert, 2003)

• Dasgupta investigated cost of screening immigrant populations prior to and after US arrival, and case-contact screening: if only considering the treatment for latent tuberculosis infections, screening all of these resulted in a cost savings for future TB case prevention

Tuberculin skin testStrengths and Weaknesses

• The PPD-S has been purified and standardized

• The test is largely safe with very few allergic reactions

• The limitations of the test have been widely researched

• Test itself is inexpensive

• Many providers are already trained in the test use

• No additional supplies are needed

• In low incidence countries like the United States, many false positives occur

• TST reacts to other tuberculin in the environment and can cause false positives

• Slow results: 48-72 hours

• Requires a repeat visit to provider for diagnosis, often resulting in non-compliance of patients

• Potential bias related to test placement on patient

• Potential bias related to physician analysis of reaction

• Can be administered incorrectly leading to a false negative result

• The cost of follow up care for false-positives are high

Quantiferon-TB in a tube testBackground

Operational definitions

• Enzyme linked immunosorbent assay (ELIZA): a biochemical test used to detect antibody or antigen in a blood sample

• Interferon gamma: a type II interferon that is secreted by T lymphocytes, dendritic cells and natural killer cells.

• Approved by the Food and Drug Administration (FDA) in 2005 to diagnose both latent and active TB infection

• QTF-G is an interferon-gamma release assay (IGRA)

• Test work by measuring patient blood antigen-specific T-cell responses to structural antigenic target-6 (ESAT-6) and the culture filtrate protein-10 (CFP-10) that come from the region of difference-1 (RD-1) from the M. tuberculosis genome

• In a tube simplifies the laboratory needs compared to the original Quantiferon-TB Gold test

Quantiferon-TB in a tube testSpecificity and Sensitivity

Specificity• 80% in published studies

(CDC)• 98% in BCG vaccinated

persons (Cellestis)• The results of the QTF-

TB in tube are unaffected by vaccination due to components used in test

• The test components are largely different from most environmental Mycobacteria, specificity problems will occur less often than TST

Sensitivity• Still under question

(CDC)• As high as 90% in

confirmed TB cases (Cellestis)

Correct to test sensitivity with 100% prevalence of disease?

Quantiferon-TB in a tube testCost

• In 2003, the Centers for Disease Control and Prevention set forth a recommendation to replace Tuberculin testing with the Quantiferon

• Test total cost of QFT is about $33.67 per person (Cellestis, 2008) [Recall: Per health care worker, TST programs in health departments ranged in cost from $176-$264 and $41-$362 at hospitals (Lambert, 2003)]

• Test produces lower number of false positives, the costs to the medical care system will represent a substantial savings after initial investment (equipment, training, etc.) (Taylor, 2001)

Quantiferon-TB in a tube testStrengths and Limitations

• Test results are a simple “yes or no” no subjective interpretations

• Test Results can be compiled in shorter period of time

• No repeated physician visits • High specificity and sensitivity in low

incidence countries like the United States (Shingadia, 2008)

• M. tuberculosis complex is separate from the BCG vaccine components, and the majority of other environmental mycobacterium eliminating almost all false positives

• In industrialized countries, the test may be less expensive given high number of false positive tuberculin skin tests that need expensive follow up care

• Will not cause allergic reactions• QFT-TB in tube was found to be more

effective testing method than TST in immunocompromised individuals (Kobashi, 2007)

• Test itself is expensive• Initial investment: requires

additional training of staff• May require the purchase of

new/ additional equipment• Indeterminate QTF-TB in tube

tests will need to be followed up

• Sensitivity of the test remains largely undetermined

Recommendations• The Tuberculin skin test has been an important diagnostic tool for

TB since 1890, but a better test has been needed. Federal funding should be used to phase out of use of TST testing in the United States.

• Quantiferon-TB Gold in a tube testing should replace the TST used for screening in high risk populations in the United States

• Quantiferon-TB Gold in a tube should be used to screen all applicants immigrating into the United States from high TB prevalence countries in an effort to reduce the TB burden in the United States

• Further studies focusing on Children <=5 years of age for Quantiferon-TB Gold in a tube testing should be performed

Acknowledgements

Dr. E. Brenner

Dr. Phyllis Edwards

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