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We retrospectively analyzed the records of 61 hospitalized patients with community-acquired pneumonia (CAP) caused by Streptococcus pneumoniae or Legionella pneumophila. We found that serum procalcitonin and sodium concentrations were significantly lower, and ferritin levels were significantly higher, in patients infected with L. pneumophila than in those infected with S. pneumoniae. The ratio of C-reactive protein to procalcitonin significantly distinguished between the groups. High procalcitonin levels were associated with an adverse clinical course.
Community-acquired pneumonia (CAP) may be caused by either Streptococcus pneumoniae or Legionella pneumophila (5, 19). The initiation of adequate empirical antimicrobial therapy can be challenging, since S. pneumoniae and L. pneumophila have partly contrasting antimicrobial susceptibility patterns (11). We thus questioned whether the determination of C-reactive protein (CRP) and procalcitonin (PCT) levels in serum could be helpful for the differential diagnosis of S. pneumoniae or L. pneumophila infection. While determination of CRP levels can be helpful for the differentiation between viral and bacterial pulmonary infections (7), elevated PCT levels have been linked to a poor prognosis in CAP and specifically in Legionella pneumonia (9, 15). However, while PCT was linked to the severity of the underlying cause, CRP was more closely linked to the presence of infections in a prospective study (8).
We retrospectively analyzed the records of 61 patients admitted to the University Hospital of Innsbruck (Innsbruck, Austria) between 2005 and 2008 with CAP caused either by S. pneumoniae (n = 37) or by L. pneumophila (n = 24). Pneumonia was diagnosed on the basis of clinical criteria and laboratory evidence of infection and was confirmed by radiographic evidence. For species diagnosis, urinary antigen tests for S. pneumoniae or L. pneumophila, the latter detecting serotype 1 (BinaxNOW; Inverness Medical, ME), were used (1, 4, 5, 10, 17).
Baseline laboratory parameters were determined by routine automated tests, and data were available from day 1 (admission) and days 5 to 7. PCT levels were determined by a time-resolved amplified cryptate emission technology assay (Brahms AG, Henningsdorf, Germany) (2).
All statistical analyses were performed with the SPSS statistical package (version 11.5; SPSS, Chicago, IL). For nonnormally distributed data. nonparametric tests were applied (Mann-Whitney test). The Spearman rank correlation technique was used for analysis of associations. All tests were two-sided, and a P value of <0.05 indicated statistical significance. Binary logistic regression analysis was used to identify parameters predictive of the patient outcomes. The ratio of CRP concentrations to PCT concentrations was calculated, and the logarithmic values were used to identify cutoffs for discrimination between L. pneumophila and S. pneumoniae infections. The diagnostic value of these cutoffs was evaluated by calculating sensitivity, specificity, and positive and negative predictive values (PPV and NPV).
While no statistically significant differences with regard to baseline demographical data became evident between CAP patients suffering from L. pneumophila versus S. pneumoniae infections (see Table S1 in the supplemental material), patients with L. pneumophila pneumonia had significantly lower PCT levels at admission (mean ± standard error of the mean [SEM], 6.76 ± 1.74 μg/liter) than patients with S. pneumoniae pneumonia (20.94 ± 3.99 μg/liter) (P < 0.01) (Fig. (Fig.1A).1A). Although CRP levels were not significantly different between the two groups, a log CRP/PCT ratio resulted in excellent discrimination between L. pneumophila- and S. pneumoniae-infected patients (P < 0.001) (Fig. (Fig.1B).1B). A log CRP/PCT ratio below 0.5 was rather indicative of S. pneumoniae infection (NPV, 54.1%; PPV, 83.3%; sensitivity, 54.1%; specificity, 83.3%), whereas a log CRP/PCT ratio greater than 1.25 was more likely to reflect L. pneumophila infection (NPV, 66.7%; PPV, 61.5%; sensitivity, 33.3%; specificity, 86.5%).
Moreover, we observed significant differences in concentrations of sodium (132 ± 1.0 mmol/liter for L. pneumophila versus 135 ± 1.0 mmol/liter for S. pneumoniae; P < 0.02), urea (52.2 ± 7.1 mg/dl versus 70.5 ± 5.8 mg/dl; P < 0.01), and ferritin (1,372 ± 303 μg/liter versus 929 ± 496 μg/liter; P < 0.01) between the two patient groups at baseline (see Table S1 in the supplemental material). Patients who died had persistently high levels of PCT (means ± SEM, 20.1 ± 10.3 μg/liter for patients who died versus 1.4 ± 0.5 μg/liter for survivors; P < 0.001) and CRP (13.3 ± 2.3 mg/dl for patients who died versus 8.2 ± 1.3 mg/dl for survivors; P < 0.02) (Table (Table1)1) at day 5.
During the observation period, 30 of 61 patients were admitted to the intensive-care unit (ICU) for invasive or noninvasive ventilator therapy. Risk factors for ICU admission were high CRP levels on day 5 (mean ± SEM, 12.3 ± 1.7 mg/dl), high PCT concentrations (25.7 ± 4.6 μg/liter on day 1; 10.7 ± 5.2 μg/liter on day 5), impaired renal function, thrombopenia, and anemia (see Table S2 in the supplemental material). Subjects with PCT levels above 10 μg/liter at admission (NPV, 77.8%; PPV, 84%; sensitivity, 72.4%; specificity, 87.5%) or with a log CRP/PCT ratio of <0.5 at admission (NPV, 73%; PPV, 79.2%; sensitivity, 65.5%; specificity, 84.4%) were more likely to be admitted to the ICU than patients with a PCT level of <10 μg/liter and/or a log CRP/PCT ratio of >0.5.
We determined that increased PCT levels at days 1 and 5 were also associated with an increased risk of death (P < 0.05 in both cases), and multivariate logistic regression analysis indicated that thrombopenia at day 5 and advanced age were risk factors for death.
Patients with L. pneumophila infections requiring ICU treatment experienced longer hospital stays (P < 0.001), whereas patients with S. pneumoniae pneumonia who were admitted to the ICU had a significantly lower survival rate (P < 0.05) than patients with an uncomplicated course of disease.
An algorithm for differential diagnosis between S. pneumoniae and L. pneumophila infection may benefit further from the inclusion of other parameters, such as serum sodium levels, since hyponatremia was often observed in patients with L. pneumophila pneumonia (see Table S1 in the supplemental material) (6, 12, 13, 20). Most strikingly, patients with L. pneumophila infection presented with significantly increased levels of ferritin, which is the major circulating iron storage protein. This is of interest because Legionella species are highly dependent on a sufficient supply of iron (3). However, ferritin is also induced by inflammation, and iron restriction by monocytes, with a subsequent increase in ferritin levels, is a cornerstone for the development of anemia of chronic disease (18). Thus, intracellular versus extracellular pathogens may cause contrasting regulatory effects on iron homeostasis (14, 16), leading to these observed differences in ferritin expression.
In conclusion, our data link PCT levels to the severity of CAP and further suggest that CRP and PCT in combination are of value for the differential diagnosis between L. pneumophila and S. pneumoniae infections. Subsequent studies involving higher patient numbers may aid in finding cutoffs for clinically useful predictive values.
This study was supported by Austrian Research Funds grant FWF-P19664 and by the “Verein zur Förderung von Forschung und Weiterbildung in molekularer Immunologie und Infektiologie an der Medizinischen Universität Innsbruck.”
Published ahead of print on 10 March 2010.
†Supplemental material for this article may be found at http://jcm.asm.org/.