Biomarkers in the Diagnosis of Mycobacterium tuberculosis Complex Infections 199
the pathogen via microscopy, culture or by PCR
where the DNA is amplified. The presence of a
sustained T-cell reactivity with M. tuberculosis
complex antigens (tuberculin skin tests) or by
use of interferon release assays of peripheral
blood is also used to determine infection (Wallis
et al., 2010). The gold standard for TB testing,
culture, is lengthy due to the fastidious nature of
MTB complex organisms with very slow genera-
tion time (20–22 hours for M. tuberculosis).
Identification of M. tuberculosis therefore takes
weeks and this delay also pushes back treatment
of ill persons who may be actively infecting
others. IGRAs are also used to detect infection,
but they only have moderate predictive value,
marginally higher than that of tuberculin skin
test in low/middle income countries (Leung
et al., 2013). IGRAs work by measuring the
IFN-γ released by T cells in a blood sample after
re-stimulation with specific M. tuberculosis
complex (MTC) antigens. A positive outcome for
IGRA gives an indication of infection but it
cannot distinguish between active and latent TB.
Despite new automated molecular ways for
TB detection and drug resistance, a simple,
affordable point-of-care test for TB is still not
available (Wallis et al., 2010). Low sensitivity is
one of the biggest challenges facing microscopy,
and one may miss diagnosis in more than 30%
of samples tested. Research shows that use of
matrix-assisted laser desorption ionization-time
of flight (MALDI-TOF) mass spectrometry and
nucleic acid amplification tests may soon accel-
erate this step for the identification of positive
cultures (Wallis et al., 2010). The current molec-
ular test (Xpert MTB/RIF) recommended by the
World Health Organization has a sensitivity of
99.7% and specificity of 98.5% in smear- positive
samples and is 76.1% sensitive and 98.8% spe-
cific in smear-negative samples (Boehme et al.,
2010). The Xpert MTB/RIF test is expensive and
not widely available in resource-limited settings
where it is needed the most. In addition, it is not
useful for testing extra-pulmonary manifesta-
tions of human and bovine TB (Wallis et al.,
2010; Gardiner and Karp, 2015).
Biomarkers are unique because they pro-
vide prognostic information about the future
health status of an individual; they can indicate
normal or pathological states, as well as
responses to anti-tubercular drug therapy. In TB
diagnostics, biomarkers are required to detect
active disease and latency as well as predict
non-relapsing treatment success in humans. In
addition, they would be useful in determining
individuals that are protected from TB by new
vaccines. Sputum-based biomarkers play a
limited role in latent TB.
A simple, non-invasive test using urine,
saliva or serum that can serve as both a diagnos-
tic and prognostic test would greatly enhance TB
diagnostics. Biomarkers in urine like lipoarabi-
nomannan (a 17.3-kDa immunogenic glycolipid
component of the mycobacterial cell wall or
LAM) have been tested with varying results as
well as detection of volatile organic compounds
in patients with pulmonary tuberculosis though
much more research needs to done to establish
changes in these biomarkers during treatment
or clinical outcome (Boehme et al., 2010). There
is currently a commercially available urine LAM
test, but use is limited due to sensitivity issues;
however, it is useful if used in combination with
other current testing methods (Leung et al.,
2013; Lamont et al., 2014b; Gardiner and Karp,
2015).
Analyses to detect antibodies against anti-
gen 85 have been carried out in blood and urine
by mass spectrometry with promising results
(Young et al., 2014). But in general, antibody
analyses have not been very effective mainly due
to the heterogeneity of the antibody response to
M. tuberculosis and thus they have not been able
to meet the requirements for a diagnostic test
(Gardiner and Karp, 2015). Data from several
researchers have concluded that antibody
responses are unlikely to provide useful diagnos-
tics for TB (Gardiner and Karp, 2015).
There are biomarkers that are increased at
baseline in proportion to the degree of the dis-
ease and subsequently decline with treatment:
these include soluble intercellular adhesion
molecule (sICAM), C-reactive protein, soluble
urokinase plasminogen activator receptor and
procalcitonin (Eckersall and Bell, 2010; Wallis
et al., 2010). Assays with these biomarkers are
simple, affordable and can be carried out on fro-
zen plasma samples so they can be incorporated
into treatment protocols. Studies indicate that
they have greatest prognostic value when mea-
sured at or near the completion of therapy
( Wallis et al., 2010). Use of a panel of biomark-
ers gives a better response than just one marker.
In addition, measuring multiple parameters