Tuberculosis in children: New diagnostic blood tests

Question 1

What are indications for the tuberculin skin test in children?

Answer 1

1. Contacts of known cases of active TB
2. Children with suspected active TB disease
3. Children with known risk factors for progression of infection to disease
4. Children travelling or residing for three months or longer in an area with a high incidence of TB, especially if the visit is anticipated to involve contact with the local population
5. Children who arrived in Canada from countries with a high TB incidence within the previous two years

Question 2

Which groups are at risk for false negatives (poor sensitivity) for the TST?

Answer 2

1. Young children
2. Infants <3mo
3. Immunocompromised patients

Question 3

Which groups are at risk for false positives (poor specificity) for the TST?

Answer 3

1. Previous vaccine with BCG

2. Infected with non-TB mycobacterium

Question 4

What interferon gamma release assays (IGRA) are available in Canada?

Answer 4

1. QuantiFERON-TB Gold In-Tube Assay

2. T.SPOT-TB test

Question 5

What does the QuantiFERON-TB Gold In-Tube assay measure?

Answer 5

Whole blood ELISA, which measures the interferon-gamma concentration in blood after incubation with the tuberculosis-specific antigens ESAT-6, CFP-10 and TB7.7. Response to both mitogen stimulation and a control are also measured to ensure quality of the assay

Question 6

What are specimen and processing requirements for the QuantiFERON-TB Gold In-Tube assay?

Answer 6

Requires the collection of 3 mL of blood, 1 mL per tube (antigen, mitogen, nil). Samples must be processed within 12 h of collection. These tubes are incubated for 16 h to 24 h, after which time the amount of interferon-gamma produced is measured via ELISA

Question 7

How do you interpret the results of the QuantiFERON-TB Gold In-Tube assay?

Answer 7

Reported as:
1. Positive (>0.35IU/mL, >50% above controls)

2. Negative (<0.35IU/mL)
3. Indeteminate (<0.35IU/mL and suboptimal mitogen response of <0.5IU/mL or measured interferon-gamma of <0.7 IU/mL

Question 8

What does the T.SPOT-TB test measure?

Answer 8

Exvivo enzyme-linked immunoassay, which estimates the number of T cells producing interferon-gamma after exposure to Mycobacterium tuberculosis-specific antigens. Isolated peripheral blood mononuclear cells from the patient are placed into microlitre wells, panel A (ESAT-6), panel B (CFP-10), nil control or mitogen, and then incubated to allow stimulation of any sensitized T cells present. The number of T cells producing
interferon-gamma are then counted and quantified

Question 9

What are the specimen and processing requirements for the T.SPOT-TB test?

Answer 9

The T.SPOT–TB assay requires 8 mL of blood for children older than 10 years of age, 4 mL for children between two and 10 years of age, and 2 mL for children younger than two years of age (with double the volume requirement for immunocompromised patients). Samples must be processed on the same day (within 8 h of collection)

Question 10

What are the interpretation of results for T.SPOT-TB test?

Answer 10

Results are presented as the number of spot-forming T cells (SFCs), and classified as either positive, negative, borderline or indeterminate. A result is defined as positive when either panel A or B contains eight or more SPCs more than the negative control, negative when both panel A or B contains four or less SPCs than the negative control, and borderline, when the highest of panel A or B contains five, six, or seven SPCs more than the negative control. A borderline test is considered equivocal and should be repeated

The presence of a satisfactory reaction (greater than 20 SFCs) to the mitogen-positive control demonstrates T cell function and validates the assay result. An indeterminate result is reported when high background levels prevent interpretation (greater than 10 SFCs in nil/control wells), or when less than 20 SFC are detected in the mitogen-positive control wells

Question 11

What is the Canadian TB committee recommendations on the use of IGRAs in children?

Answer 11

1. For close contacts of an index case or contacts with increased rate of disease progression a TST (or TST and IGRA) should be used 8-12wk from most recent exposure
2. The IGRAs may be used as a supplementary diagnostic aid in combination with the TST to help support the diagnosis of active TB.
3. In the absence of a positive TST or culture confirmation, a positive IGRA could potentially support the diagnosis of TB based on typical clinical, radiological or other laboratory findings. However, the IGRA must never be used in isolation to make a diagnosis of TB disease. Every effort must still be made to obtain a microbiological confirmation of active TB. A negative IGRA test (or TST) does not rule out active TB at all ages, but especially in young children.
4. The IGRAs may be used in the setting of contact investigation to confirm a positive TST in contacts who, on the basis of an assessment of the duration and degree of contact with an active infectious case, are believed to have a low pretest probability of recently acquired LTBI, and who have no other high or increased risk factors for progression to active disease if infected.
5. The IGRAs may be performed in a TST-positive, immunocompetent child with relatively low risk of being infected with TB, and of progressing to active disease if infected. Persons with a positive IGRA result may be considered for treatment of LTBI.
6. Routine or mass screening of all immigrant children for LTBI, with either TST or IGRA, is not recommended. However, targeted screening for LTBI after arrival in Canada is recommended for foreign-born children and travellers with risk factors for reactivation of LTBI.
7. The IGRAs can be used, in addition to the TST, to diagnose LTBI infection in an immunocompromised patient.

Question 12

What are risk factors that result in being high risk for development of active TB?

Answer 12

1. AIDS
2. HIV infection
3. Transplantation (related to immunosupression therapy)
4. Silicosis
5. Chronic renal failure requiring hemodialysis
6. Carcinoma of head and neck
7. Recent TB infection (<2yo)
8. Abnormal CXR (fibronoduclar disease)

Question 13

What are risk factors that result in being at increased risk for development of active TB?

Answer 13

1. Treatment with glucocorticoids
2. TNF alpha inhibitors
3. DM (all types)
4. Underweight
5. Young age when infected (0-4yo)
6. Cigarette smoker
7. Abnormal CXR- granuloma