Joseph P. Iser, MD, DrPH, MSc Chief Health Officer

Clinical Updates on Tuberculosis and STDsSeptember 30 – October 1, 2019Las Vegas, NV

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Joseph P. Iser, MD, DrPH, MSc

Chief Health Officer

There are two kinds of tests that are used

to determine if a person has been

infected with TB bacteria• The tuberculin skin test and

• TB blood tests (IGRAs)

These do not differentiate between latent

TB infection and active TB disease

They also do not predict who may

progress from latent to active disease



Patient history and other tests are

needed to determine whether the person

has TB disease• Medical history

• Physical examination

• Test for TB infection

• Chest x-ray

• Appropriate laboratory testing (sputum

samples)

Those who have spent time with someone

with TB disease

People from countries where TB is

common

People who work or live in high-risk

settings

Healthcare workers exposed to TB

Some others



People with HIV

Recently exposed

Young children

Injection drug users

Other immune deficiencies

Elderly

Inadequately treated people

Tuberculin Skin Test• Performed by injecting a small amount of fluid

(tuberculin) subcutaneously

• Person must return within 48-72 hours to read so requires 2 visits

• Look for raised hard area (induration) and measure size by ruler, compare to standards

• Redness makes no difference to reading

• Size of reaction is an estimate of risk for TB infection

• Preferred in children younger than 5



Tuberculin Skin Test—Positive• Positive Skin Test means that the person’s body

was infected with TB bacteria

• Additional tests are required to determine

whether the infection is latent or active

Tuberculin Skin Test—Negative• This means the person’s body did not react

• TB infection or disease is not likely (with

exceptions)

TST preferred in young children



Also called interferon-gamma release

assays (or IGRAs)

Measure how strong the immune system

reacts to TB bacteria by testing the

person’s blood in a laboratory

Two IGRA tests are approved by FDA and

available in the US• QFT Gold Plus (used by SNHD)

• T-SPOT TB Test (T-Spot)

Either requires only one visit

Measure a person’s immune reaction to

M tuberculosis



Positive IGRA• Person has been infected with TB bacteria

• Does not differentiate between latent and active cases

• Additional tests are needed to determine whether the diagnosis is latent or active TB

• Treatment would be done by a HCW

Negative IGRA• The person’s blood did not react to the test

• Latent TB infection or active TB not likely

IGRAs are the preferred method of TB

infection testing for• People who have received bacille Calmette-

Guerin (BCG); BCG is a TB vaccine

• People who have a difficult time returning for a

second appointment to look for a reaction to the

TST



IGRA interpretations based on the

amount or number of cells releasing IFN-

gamma

IGRAs are an aid to diagnosis

Indeterminate or borderline tests are just

that

Diagnosis requires medical evaluation

and epidemiological and historical

information

Generally NOT needed for people with a low risk of infection

Those at higher risk should be tested• People who have spent time with an active case

• People with immune deficiencies

• People with symptoms

• People from countries where TB is endemic

• People who live or work in congregate living situations

• People who use illicit drugs



Many people born outside the US have

been vaccinated

They may receive a TST, but BCG may cause

a positive skin test when they have not been

exposed to TB bacteria (false-positive)

If positive TST, additional tests are needed

IGRAs are NOT affected by BCG vaccination

and should not have a false-positive result

Health care provider should choose

which test to use

Factors to consider include the reason for

testing, test availability, and cost

Costs for IGRAs can be ~$35 (QFT) to $55

(T-SPOT)

Generally, it is not recommended testing

a person with multiple types of tests



Requires a single patient visit to conduct

the test

Results can be available within 24 hours

Does not boost responses measured by

subsequent tests

Prior BCG vaccination does not cause a

false-positive IGRA test result

Blood samples must be processed within 8-30 hours after collection while white blood cells are still viable

Errors in collecting or transporting blood specimens or in running and interpreting the assay can decrease the accuracy of IGRAs

Limited data on the use of IGRAs to predict who will progress to TB disease in the future



There is limited data on the use of IGRAs

for• Children younger than 5 years of age

• Persons recently exposed to M tuberculosis

• Immunocompromised persons

• Serial testing

Tests are expensive

If infection is shown by a positive TB test,

other tests are needed to determine whether

the person has active TB

History, PE, and other tests have outline in

slides above

If the person does not have active disease

but only latent infection, treatment options

should be based on a person’s chances of

developing TB disease



Increased risk for acquiring TB from

patients with unrecognized TB disease

Increased risk of transmission to

vulnerable/immunocompromised

patients

Annual testing is not recommended for

HCW not at high risk

Greater risk to pregnant women and

babies

TST valid and safe

IGRA valid and safe

Joseph P. Iser, MD, DrPH, MSc Chief Health Officer

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