Frequently Asked Questions
What is Tuberculosis
(TB)?
Tuberculosis (TB) is a preventable & curable airborne
infectious disease mainly caused by the bacterium Mycobacterium
tuberculosis.
How does TB infection
occur?
Infection occurs through inhalation of airborne TB bacteria with
repeat exposure generally necessary for infection to take
place.
What is latent TB
infection?
In most cases (90%) the immune system walls off the bacteria and
stops it from growing once infected.1
In these cases the infection lies dormant, asymptomatic and is
termed as latent TB infection.Those with latent TB feel well and
cannot pass the infection onto others.
What is active TB?
There is a 10% lifetime chance that a latent TB infection will
progress to TB disease (active TB).1
Active TB occurs when the TB bacteria overcome the immune system
defenses and begin to multiply. Active TB is described in two
forms: pulmonary (80-90% of cases) and extrapulmonary (10-20% of
cases).1
What is pulmonary
TB?
Pulmonary is localised in the lungs and is the most common form
of active TB disease. Patients with untreated pulmonary
TB are highly infectious. On average, each person with
untreated pulmonary TB will infect 10-15 people a year.
Without treatment by the end of 5 years: 50% of pulmonary TB
patients will be dead, 25% will be healthy (self-cured by strong
immune defense) and 25% will remain ill with chronic infectious
TB.2
What is extrapulmonary
TB?
At infection, TB bacteria may spread through the blood,
lymphatic or GU systems to parts of the body outside the lungs. As
in the lungs, the TB infection can lay dormant for months or
years. Extrapulmonary TB occurs when active
tuberculosis infection occurs at body sites other than the lungs.
Where extrapulmonary TB and pulmonary TB are
present a patient is usually classified as pulmonary TB. Symptoms
are related to the organs in which the disease occurs and may
manifest as a condition e.g. fever or fatigue.
What is the link
between HIV and TB?
TB is a leading cause of death in HIV infected patients. TB and
HIV have a mutually negative relationship. As HIV progresses
CD4+ cell counts drop, the co-infected patient suffers from
immunodeficiency, which leads to increased susceptibility to active
TB (up to 10x more likely than someone that is HIV
negative).2 HIV is driving the TB epidemic in many
countries such as Sub-Saharan Africa (up to 50% of TB patients in
African countries are also HIV positive), Asia and South
America.3 Extrapulmonary TB is more common in those
co-infected with HIV as the ‘immune system becomes less able to
prevent growth and local spread’.2
What is Clearview TB
ELISA?
The Clearview TB ELISA kit is an enzyme-linked
immunosorbent assay (ELISA) system used to qualitatively detect the
presence of lipoarabinomannan (LAM) antigen of mycobacteria in
non-concentrated human urine.
What is lipoarabinomannan
(LAM)?
Lipoarabinomannan (LAM) antigen is a cell wall
lipopolysaccharide specific to the genus Mycobacterium. It
is released from metabolically active or degrading bacterial cells.
Once in the bloodstream, LAM is filtered by the kidneys and can be
detected in urine intact.4
Why
detect LAM antigen and not LAM antibodies?
Studies have shown that the use of LAM antibodies in the
detection of active Mycobacterium
tuberculosis can lead to inaccurate and misleading
results.5 Detection of LAM antibodies
also poses a challenge in those patients where the immune response
has been compromised, such as in those TB patients co-infected
with HIV. In contrast, detection of LAM antigen provides a
Mycobacterium specific target and is readily detectable in patients
co-infected with TB-HIV.6
Why
do TB-HIV co-infected patients have elevated levels of LAM antigen
in their urine?
In TB-HIV patients the inability of the body’s immune system to
fight the spread of the infection results in high levels of
bacteria in the blood which are metabolized by the kidneys and
result in filtration of complete LAM antigen into the
urine.4 Therefore, in TB-HIV patients
it is possible to directly detect these elevated levels of LAM
antigen in the urine. However, in patients with active TB who are
not infected with HIV, LAM levels are below or at the low end of
the assay’s detectable range due to the body’s immune response
against TB.4
How does Clearview TB
ELISA detect LAM antigen?
Antibodies adsorbed on the ELISA plate capture the carbohydrate
surface antigen found in positive test samples. The conjugated
antibodies then attach to the captured antigen creating a sandwich
assay. In the presence of the color developer, a color change
occurs. The assay reaction is stopped using the stop solution, and
the intensity of the color (optical density) is measured using a
microtiter plate reader. A positive result indicates that LAM
antigen of mycobacteria is present in the sample, whereas a
negative result indicates that it is not present at or above the
test’s detection limits.
Does Clearview TB
ELISA detect latent TB infection?
No. Clearview TB ELISA does not detect latent
TB infection.
Does
Clearview TB ELISA detect active pulmonary TB?
Clearview TB ELISA specifically detects LAM
antigen derived from bacteria in the patient’s blood which
have been metabolized by the kidneys and passed into the
urine.6,4 As such, a
patient with pulmonary TB may produce sufficient levels
of LAM to be detected by the Clearview TB ELISA
test. Co-infection with HIV and pulmonary TB promotes the
spread of the TB from the lungs into the bloodstream due to the
reduced ability of the body’s immune system to fight the infection.
Therefore, in those patients with pulmonary TB and HIV,
levels of LAM are likely to be elevated when compared to those
uninfected with HIV.
Does
Clearview TB ELISA detect active extra pulmonary TB?
Clearview TB ELISA specifically detects LAM
antigen derived from bacteria in the patient’s blood which have
been metabolized by the kidneys and passed into the
urine.6,4 As such, a
patient with extrapulmonary TB may produce sufficient
levels of LAM to be detected by the Clearview TB
ELISA test. Co-infection with HIV and extrapulmonary
TB promotes the spread of the TB bacteria to other locations
within the body, and its entry into the bloodstream due to the
reduced ability of the body’s immune system to fight the
infection.2 Therefore, in those patients
with extrapulmonary TB and HIV, levels of LAM are likely
to be elevated when compared to those uninfected with HIV.
Do urine
samples need to be collected in containers containing
preservatives?
No. Urine samples need only be collected in a standard urine
collection container.
Is it necessary to boil the
urine sample before testing?
All samples must undergo the recommended heating and
centrifugation step due to the infectious nature of patient samples
and in order to ensure optimal assay performance and reduce
variability.
Is it necessary to run samples
in duplicate?
In order to minimize the risk of abnormal sample
readings, all controls and samples must be run in
duplicate and an average absorbance result calculated. OD values
for replicates of the same sample must be within 15% of the average
signal.
How critical is the rate at
which samples are added to the plate?
In order to minimize variability in sample and control reaction
times across the plate, controls and samples should be added at a
consistent rate. During the course of the assay any additions to
the controls or sample wells (Conjugate, TMB, and Stop solution)
should also be performed at a consistent rate.
Is it possible to perform the wash
steps manually?
Yes. Correct wash procedures are necessary in order to obtain
correct results. Inadequate washing may result in inaccurate
results. Full instructions for manual wash procedure can be found
within the product insert.
Does a positive result mean that the
patient has active TB?
In areas endemic for tuberculosis, the LAM detected in a
clinical sample is likely to be attributed to M.
tuberculosis. However, Clearview TB ELISA
does not differentiate between the various species of
mycobacterium, such as M. tuberculosis, M. leprae, and
M. avium. Therefore Clearview TB ELISA
should be followed up with a confirmation test such as bacterial
culture.
Does a negative result mean that the
patient does not have active TB?
A negative test result does not exclude infection with M.
tuberculosis as it is possible that patients may have LAM
antigen concentrations below the detection limits of this test.
References
1. WHO Diagnosis for Tuberculosis: Global demand & market
potential (2006)
2. WHO/HTM/TB/2004.329: TB/HIV A Clinical Manual 2nd
Ed.(2004)
3. WHO
TB-HIV: Facts at a Glance
4. Boehme C, Molokova E, Minja F, Geis S, Loscher T, Maboko L,
Koulchin V, Hoelscher M. Detection of mycobacterial
lipoarabinomannan with an antigen-capture ELISA in unprocessed
urine of Tanzanian patients with suspected tuberculosis. Trans R
Soc Trop Med Hyg. 2005 Dec;99(12):893-900.
5. Tessema TA, Bjune G, Hamasur B, Svenson S, Syre H, Bjorvatn
B. Circulating antibodies to lipoarabinomannan in relation to
sputum microscopy, clinical features and urinary
anti-lipoarabinomannan detection in pulmonary tuberculosis. Scand J
Infect Dis. 2002;34(2):97-103.
6. See Package
Insert