Keywords |
| Procalcitonin, CRP, Tuberculosis, Calcium, Phosphorus, Infection |
Introduction |
| Procalcitonin (PCT), a propeptide of the hormone calcitonin, is a novel marker of the
inflammatory response to infection. It has been used to discriminate between infectious and
non-infectious causes of inflammation, and as a marker of severe sepsis in the intensive care
unit.1 However, there have been contradictory reports by researchers where the utility of PCT
in diagnosing pulmonary tuberculosis is concerned. While some studies implicate a poor
diagnostic value to PCT in lower respiratory tract infections like pulmonary tuberculosis,2
some others report that PCT could be a good indicator of inflammation in patients with
chronic diseases and in persons exposed to long-lasting infections like tuberculosis.3 |
| Researchers have also concluded that PCT is a useful biomarker for discriminating between
pulmonary tuberculosis and community acquired pneumonia (CAP) because elevation of
PCT in tuberculosis was not as significant as in CAP.4 It is well known that during host
defense, neutrophil-mediated killing of mycobacteria is a Ca2+-dependent process.5
Malik ZA et al6 have demonstrated that M. tuberculosis inhibits CR-mediated Ca (2+)
signaling in a bid to inhibit phagosome-lysosome fusion and promote intracellular
mycobacterial survival. Thus, the contradictory reports of diagnostic utility of PCT could be
due to variations in the severity of the disease with respect to host immunity status. Also,
measurement of alteration in serum PCT levels in conjunction with that in serum calcium
levels could be useful in the diagnosis of pulmonary tuberculosis. This study is aimed to
assess the combined diagnostic value of serum PCT and serum calcium in pulmonary
tuberculosis patients, in the background of changes in serum CRP which is a well-known
acute phase protein as well as a non-specific marker of systemic inflammation and serum
phosphate which is known to be closely associated with serum calcium. |
Subjects and Methods |
| The study subjects were recruited from The Institute of Thoracic Medicine, Chennai, India.
The study group consisted of 40 patients (25–75 yrs.) diagnosed with pulmonary
tuberculosis. The diagnosis of tuberculosis was performed using Ziehl-Neelsen staining method for Acid-fast Microscopy (AFM)7 and culture for growth of the organism on
Lowensterin-Jensen (LJ) medium.8 The patients were also tested for radiographic
abnormalities. Age and sex matched healthy volunteers (n=40) were included in the study as
normal controls for comparison of results. The study protocol was approved by the
Institutional ethics committee and was carried out in accordance with the principle of
Declaration of Helsinki. Informed consent was obtained from all the subjects. |
| Blood samples were collected immediately following diagnosis using standard sampling
tubes. Ten milliliter of venous blood was drawn and transferred to a centrifuged tube without
any anticoagulant. The blood sample was allowed to clot for about one hour and subjected to
centrifugation at 3000 × g for 15 min at room temperature and serum was separated. All the
experiments were performed within 8 hours of sample collection. |
| PCT measurements were done using semi-quantitative PCT-Q kit (B.R.A.H.M.S. Diagnostica
GmbH, Berlin, Germany) which employs a monoclonal mouse anti-catacalcin antibody
conjugated with colloidal gold (tracer) and poly sheep anti-calcitonin antibody (solid phase)
to form a sandwich complex with PCT in the test sample. The color intensity of the band thus
formed is directly proportional to the PCT concentration in the sample and is expressed as
four grades of > 0.5 ng/ml, 0.5-2 ng/ml, 2-10 ng/ml and > 10 ng/ml with the help of the
reference card. |
| CRP level was determined by immunoturbidimetic assay.9 Calcium was analyzed by
photometric test using Arsenazo III end point.10 and phosphorus was determined by
phosphomolybdate method.10 |
Statistical analysis |
| Statistical analysis was made by the students’ T-test. A p value of less than 0.05 was
considered significant, less than 0.01 very significant and less than 0.001 as extremely
significant. |
Results |
| Results of PCT estimation are shown in Figure 1. Serum PCT levels in thirty seven out of
forty normal controls had were <0.5 ng/ml and three showed PCT values of > 2 ng/ml.
However, PCT levels in fifteen out of forty TB patients were > 2ng/ml and the rest of the
patients showed PCT levels > 10 ng/ml, thus indicating that PCT levels served as a useful
marker of infection in TB patients. |
| As shown in Table 1, the increase in serum CRP levels and the decrease in serum calcium levels was extremely significant (p<0.001) in TB patients when compared to normal controls.
Serum phosphate levels were significantly lower in TB patients (p<0.01) when compared to
normal controls. Serum CRP, calcium and phosphate levels of the three normal controls who
recorded PCT values > 2 ng/ml, were in the range of values recorded by normal controls
(Table 2). These results indicate that measurement of serum PCT in conjunction with serum
calcium and phosphate levels could serve as a useful diagnostic tool in tuberculosis. |
Discussion |
| Mycobacterium tuberculosis has been rated as the leading cause of mortality due to an
infectious disease.11 Despite aggressive research conducted on this disease and its
mechanism, the question still remains,” how to control the disease”? The presence of reliable
diagnostic markers is an important factor contributing to the successful treatment of any
disease. Serum PCT has been reported as a useful biomarker for diagnosis and prognosis of
CAP by several researchers.1,4,12,13 However, most studies 2,14,15 have reported that serum PCT
was not elevated significantly in pulmonary tuberculosis. Baylan et al16 has reported that PCT
level was not a reliable indicator in the diagnosis of active PTB because of its low sensitivity
(41.3%), and PCT test for the presumptive diagnosis of PTB cannot be a substitute for
microbiological, epidemiological, clinical and radiological data. However, Ozlem K et al3
report that PCT could be a good indicator of inflammation in tuberculosis. These
contradictory reports have made it necessary to understand the biochemical mechanism of
elevation of PCT in bacterial infections. Increased efforts in tuberculosis research are being
aimed at better defining the interactions between the causative bacterium and the host
immune system. Since PCT synthesis and release are determined by the inflammatory
cytokine cascade during systemic infection, the intensity depends on the number of
organisms entering the systemic circulation.4 The number of organisms entering circulation
in TB patients depends on the severity of the disease which in turn depends on the
effectiveness of the host defense. Mycobacterium tuberculosis evades the innate
antimicrobial defenses of macrophages by inhibiting the maturation of its phagosome to a
bactericidal phagolysosome. Phagosome formation triggers a preprogrammed pathway of
maturation into the phagolysosome, a process controlled by Ca2+.17,18 In the present study,
the decreased serum calcium levels in tuberculosis patients indicates a decreased availability
of calcium for phagolysosome maturation, decreased efficiency of host antimicrobial activity
and hence increased severity of the disease. Although there were three healthy controls with
PCT > 2 ng/ml, serum calcium levels in them were well within normal range. This indicates
that simultaneous measurement of serum calcium could increase the diagnostic utility of PCT
in tuberculosis patients. |
Conclusion |
| Although currently available research does not validate the diagnostic utility of PCT in
tuberculosis patients, results of the present study indicate that measurement of serum PCT
along with serum calcium could prove as a useful diagnostic marker for the disease. The
findings imply that it is imperative to crack the underlying mechanism of increase in PCT
during bacterial infections (namely- Why bacterial infections induce the PrePCT gene? Is
PrePCT preferentially proteolysed to PCT over the proteolysis of PCT to calcitonin, to result
in the increased serum PCT and so on) to understand and improve its diagnostic utility. |
| The present study encourages further research to validate the role of serum PCT-serum
calcium combination in differential diagnosis of latent versus active tuberculosis and mild
versus severe tuberculosis. It also calls for research at the molecular level on the relative rate
of post translational modifications of PrePCT and PCT. This could help us understand the
specific situations in which serum PCT is increased/not increased significantly in bacterial
infections |
| Conflicts of interest: None declared |
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