Clinical analysis of community-acquired pneumonia in the elderly

OBJECTIVE: To evaluate the clinical features, etiology, and outcome of patients over 65 years old hospitalized for community-acquired pneumonia. PATIENTS: Eighty-four patients (50 males, 34 females) hospitalized for community-acquired pneumonia in Kawasaki Medical School Kawasaki Hospital between April 1998 and March 2000. RESULTS: Most of the patients had respiratory symptoms or signs, but over one-third also had atypical symptoms of pneumonia such as dyspnea, consciousness disturbance, and gastrointestinal symptoms. The causative microorganisms were identified in 48% of these patients. Streptococcus pneumoniae (13%), respiratory viruses (13%), Haemophilus influenzae (8%) and Mycobacterium tuberculosis (8%) were frequently identified, but Mycoplasma pneumoniae was less frequently noted in the elderly. Double infection was recognized in 19 % and a combination of some virus and bacteria in 13%. Treatment consisted of the administration of second or third generation cephalosporin antibiotics intravenously, because antibiotics had already been preadministered in 39%. The prognosis was poor (mortality rate 9%) for the elderly with community-acquired pneumonia despite mechanical ventilation in 8%. CONCLUSIONS: Although the range of microorganisms causing community-acquired pneumonia differed slightly from that in previous reports; namely, lower frequency of Chlamydia pneumoniae and Legionella pneumophila, it is suggested that the initial antibiotic treatment should always cover S. pneumoniae and H. influenzae. In addition, since a prevalence of virus infections related to the increase in community-acquired pneumonia in the elderly was found in this study, the routine use of influenza vaccine and pneumococcal vaccines in the elderly is recommended to reduce the high mortality rate.     

Clinical analysis of pneumonia in the elderly in a community hospital--comparison of community-acquired pneumonia and nosocomial pneumonia]

We experienced 530 elderly cases with pneumonia among 930 patients with pneumonia in Kawasaki Medical School Kawasaki Hospital between April 1986 and September 1998. Clinical analysis of all these patients and a comparison of one group consisting of 418 patients with community-acquired pneumonia and another group composed of 112 patients with nosocomial pneumonia were performed. In all of the elderly patients with pneumonia, respiratory symptoms and inflammatory findings were less frequent, but were frequent for those in poor general and nutritional condition. The causative microorganism was isolated in 42% of these patients. Streptococcus pneumoniae, MSSA and Klebsiella pneumoniae were frequently isolated from the sputum of the patients with community-acquired pneumonia, while Methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa, Methicillin-sensitive Staphylococcus aureus (MSSA) were frequently isolated from that of nosocomial pneumonia patients. Mycoplasma pneumoniae, Chlamydia pneumoniae and some viruses were less frequent for patients in both groups. Although many intravenous antibiotics, such as cephem or carbapenem were administered to patients in both groups, the prognosis was relatively good for those with community acquired pneumonia but was extremely poor for those with nosocomial pneumonia despite mechanical ventilation or steroid pulse therapy for many patients.     

Clinical analysis of community-acquired pneumonia and pulmonary tuberculosis in the elderly and advances in treatment]

We treated 510 elderly case (over 65 years old) among 1,017 patients with community-acquired pneumonia and 60 similar cases among 112 patients with pulmonary tuberculosis in Kawasaki Medical School Kawasaki Hospital during approximately the past 15 years. These were compared with non-elderly cases (below 65 years old). In the elderly cases with community-acquired pneumonia, atypical clinical symptoms or physical signs were frequent and the mortality rate was high because of severe underlying diseases, and poor general and nutritional conditions. Regarding a prospective study of 84 elderly cases with community-acquired pneumonia during the past two years, S. pneumoniae, Respiratory virus, Gram-negative bacilli, H. influenzae, M. Tuberculosis were frequently isolated. In addition, mixed viral and bacterial infections, which were frequently noted during the winter, were significantly related to the increased frequency of community-acquired pneumonia. In treating elderly cases with community-acquired pneumonia, immunization therapy (e.g., influenza vaccine), second cephalosporin and/or macrolide antimicrobial agents for outpatients with mild pneumonia, and carbapenem and/or macrolide antimicrobial agents for hospitalized patients with moderate or severe pneumonia were most effective. The number of elderly cases with pulmonary tuberculosis has recently increased and the recognition of 10 cases was delayed because of a low percentage of positive smears, but no resistance to antituberculosis drugs have been observed. Regarding the treatment of pulmonary tuberculosis, fluoroquinolone and rifamycin derivative antibiotics have been developed as antituberculosis drugs with strong antituberculous activity. However, due to the high percentage of adverse effects in elderly patients, careful treatment with desensitization therapy for antituberculosis drugs is considered important.     

Predictive factors of etiologic agents of community-acquired pneumonia presenting at a district general hospital]

Community-acquired pneumonia is one of the major respiratory diseases causing hospital admission in previously healthy patients. Prompt and appropriate antibiotic selection is essential for recovery. The authors tried to determine the distribution of the etiologic agents of community-acquired pneumonias and to analyze predictive factors. Out of 188 cases of community-acquired pneumonia presenting to our hospital, etiologic agents were determined in 106 cases (56%). Twenty-nine cases were due to Streptococcus pneumoniae, 27 cases due to Mycoplasma, 17 cases due to Haemophilus influenzae and 21 cases due to Mycobacterium tuberculosis. M. tuberculosis was the cause in 11% of all cases and the importance of pulmonary tuberculosis must be emphasized as a community-acquired pneumonia. Out of 58 cases under 50 years old, Mycoplasma pneumoniae was the etiologic agent in 23 cases (40%) and S. pneumoniae in 7 cases (12%). Out of 62 cases not less than 70 years old. M. tuberculosis was the most common etiologic agent (15 cases, 24%). S. pneumoniae followed, being causative in 13 cases (21%). M. tuberculosis was the cause in 10 cases out of 31 cases who did not complain of fever at presentation. In 86 cases who did not show leukocytosis on admission, 21 cases were due to Mycoplasma (24%) and 15 cases were due to M. tuberculosis (17%). In particular 17 cases were due to Mycoplasma among 28 cases under 50 years old without leukocytosis (61%), and 11 cases were due to M. tuberculosis in the 27 cases no less than 70 years old without leukocytosis (41%).(ABSTRACT TRUNCATED AT 250 WORDS)     

Current status of patients discharging Mycobacterium tuberculosis after admission to Kawasaki Medical School Hospital

OBJECTIVES: To investigate the current status of patients with tuberculosis in Kawasaki Medical School Hospital which has closed tuberculosis isolation ward and review the nosocomial tuberculosis infection control. MATERIALS AND METHODS: Clinical analysis was performed and nosocomial tuberculosis infection control were examined in 39 patients from whom Mycobacterium tuberculosis was isolated in Kawasaki Medical School Hospital between January 2001 and August 2004. RESULTS: Mycobacterium tuberculosis was isolated in 16 patients in the Respiratory Division of the Department of Internal Medicine and in 23 patients in non-respiratory divisions. Thirty-four patients had underlying diseases and of these 14 patients had malignant diseases. The final diagnosis was pulmonary tuberculosis in 23 patients, and pulmonary tuberculosis was suspected in 13 patients on admission. The remaining 10 patients were treated for pneumonia on admission. M. tuberculosis was isolated most frequently from the sputum in 21 patients and 13 of them were smear positive who needs nosocomial infection control measures. Health examinations of the families and hospital staff in contact with these 13 patients with smear positive sputum after the diagnosis of tuberculosis revealed no active case of tuberculosis. CONCLUSIONS: This study has shown that there are many cases with an atypical pattern for pulmonary tuberculosis among patients with underlying diseases, especially malignant diseases. There are still many tuberculosis patients who were diagnosed pneumonia after the admission and were administered antibiotics. We believe that more educational guidance regarding tuberculosis is needed for the hospital staff.     

Community-acquired Chlamydia pneumoniae pneumonia in Japan: a prospective multicenter community-acquired pneumonia study

OBJECTIVE: To investigate the clinical features of Chlamydia pneumoniae pneumonia in Japan and to evaluate the newly created Japanese community-acquired pneumonia (CAP) guidelines. PATIENTS AND METHODS: A multicenter CAP surveillance study was carried out in 20 hospitals between December 1999 and March 2000. The diagnosis of C. pneumoniae infection was based on isolation in cell culture, the polymerase chain reaction and serologic testing of antibodies by the microimmunofluorescence test. RESULTS: Among 232 CAP cases, C. pneumoniae was identified as the etiologic agent in 15 cases (6.5%). C. pneumoniae was the only pathogen identified in nine of these cases, while one or more additional etiological agents were found in the other six cases. Of the present and previously reported single agent C. pneumoniae pneumonia cases, about 50% were more than 60 years old and had underlying diseases. A relatively slow pulse rate in relation to fever was not seen in these patients. The mean WBC count of all patients was normal. No patient required respiratory support or admission to an intensive care unit and no deaths occurred among these patients. CONCLUSION: The clinical pictures of C. pneumoniae pneumonia as a single agent were mild to moderate and were remarkably different from those of cases of C. pneumoniae pneumonia concomitant with other bacteria. If the patient is less than 60 years old and some guideline headings are excluded, we think it would be possible to distinguish between C. pneumoniae and bacterial pneumonia.     

A prospective study of community-acquired pneumonia in Hong Kong.

A prospective study of community-acquired pneumonia in Hong Kong was carried out between January and December, 1988. Ninety adults (57 male) with a mean age of 57.3 years were admitted to the Prince of Wales Hospital with community-acquired pneumonia. The etiologic diagnosis of pneumonia was made in 37 cases (41 percent). Pneumococcal infection was diagnosed in 11 patients (12 percent). The same number of patients had pulmonary tuberculosis presenting as acute pneumonia. It could not be differentiated from other causes of pneumonia on clinical and radiologic grounds, although pleural effusion and upper lobe involvement were more common in patients with tuberculosis. Chlamydia species were identified in five patients (6 percent) and Mycoplasma pneumoniae was identified in three patients (3 percent). There was no case of Legionnaires' disease. The etiologic agent could not be identified in 59 percent of cases. The low incidence of etiologic diagnosis of community-acquired pneumonia was probably related to the widespread use of antibiotics in private practice. Tuberculosis is an important cause of community-acquired pneumonia in Hong Kong and this diagnosis should be considered in patients who fail to respond to first-line antibiotics.     

Clinical features of community-acquired pneumonia treated at Srinagarind Hospital,Khon Kaen,Thailand

Pneumonia is a serious illness associated with significant morbidity and mortality. The interpretation guidelines for pneumonia management requires knowledge of both the clinical presentation of the disease and local epidemiology. We studied the clinical features, initial laboratory results, antibiotic sensitivities, and outcomes of patients diagnosed with acute community-acquired pneumonia between January 1999 and December 2000 at Srinagarind Hospital. The causative organisms were identified in only 52.2% patients; Streptococcus pneumoniae accounted for 23.1% of infections. Other common causes included Klebsiellapneumoniae (19.2%), Burkholderia pseudomallei (15.4%), Hemophilus influenzae (11.5%), Mycoplasma pneumoniae (6.2%), and Staphylococcus aureus (4.6%). Younger patients were more likely to be infected with M. pneumoniae, while the mean age of those with other types of infections was 50. Healthy adults were infected with M. pneumoniae and S. pneumoniae; specific pathogens attacked patients with certain co-morbidity : i) diabetes mellitus and ageing, ii) diabetes mellitus and renal disease, iii) cardiovascular diseases, and iv) connective tissue diseases and steroid-use; these patients were vulnerable to i) K. pneumoniae, ii) B. pseudomallei, iii) H. influenzae, and iv) S. aureus respectively. White blood cell counts were normal in M. pneumoniae infection. Gram-stained sputum had some limitations, especially when determining Gram-negative infections; chest x-rays could not differentiate pathogens. Bactermia was found in one half of patients infected with B. pseudomallei and S. aureus. Antibiotic-resistant organisms were not common in our study. Because morbidity and mortality were high among patients infected with S. aureus and B. pseudomallei, empirical antibiotic treatment should be considered in suspected cases, especially when patients present with acute severe community-acquired pneumonia.     

A case of pneumonia due to coinfection by Mycoplasma pneumoniae and Chlamydia pneumoniae]

Few cases of Mycoplasma pneumoniae and Chlamydia pneumoniae coinfection in pneumonia have been reported in adults. We report a case of such a double infection in a young adult. A 16-year-old boy was admitted to our hospital with dry cough and fever. Laboratory findings revealed elevated serum GOT and GPT levels. The patient had been administered a beta-lactam antibiotic before admission to our hospital. Antibodies to M. pneumoniae were significantly elevated. Titers of IgM and IgG specific for C. pneumoniae titer were high, as measured by the enzyme-linked immunosorbent assay method. The patient was treated with clarithromycin and discharged after a satisfactory recovery. M. pneumoniae and C. pneumoniae may act as cofactors in community-acquired pneumonia. Further studies are needed to clarify the relationships of these pathogens to community-acquired pneumonia.     

Etiology of community-acquired pneumonia in a population-based study: Link between etiology and patients characteristics, process-of-care, clinical evolution and outcomes

ABSTRACT: BACKGROUND: The etiologic profile of community-acquired pneumonia (CAP) for each age group could be similar among inpatients and outpatients. This fact brings up the link between etiology of CAP and its clinical evolution and outcome. Furthermore, the majority of pneumonia etiologic studies are based on hospitalized patients, whereas there have been no recent population-based studies encompassing both inpatients and outpatients. METHODS: To evaluate the etiology of CAP, and the relationship among the different pathogens of CAP to patients characteristics, process-of-care, clinical evolution and outcomes, a prospective population-based study was conducted in Spain from April 1, 2006, to June 30, 2007. Patients (age >18) with CAP were identified through the family physicians and the hospital area. RESULTS: A total of 700 patients with etiologic evaluation were included: 276 hospitalized and 424 ambulatory patients. We were able to define the aetiology of pneumonia in 55.7% (390/700). The most frequently isolated organism was S. pneumoniae (170/390, 43.6%), followed by C. burnetti (72/390, 18.5%), M. pneumoniae (62/390, 15.9%), virus as a group (56/390, 14.4%), Chlamydia species (39/390, 106%), and L. pneumophila (17/390, 4.4%). The atypical pathogens and the S. pneumoniae are present in pneumonias of a wide spectrum of severity and age. Patients infected by conventional bacteria were elderly, had a greater hospitalization rate, and higher mortality within 30 days. CONCLUSIONS: Our study provides information about the etiology of CAP in the general population. The microbiology of CAP remains stable: infections by conventional bacteria result in higher severity, and the S. pneumoniae remains the most important pathogen. However, atypical pathogens could also infect patients in a wide spectrum of severity and age.     

Differences in the etiology of community-acquired pneumonia according to site of care: a population-based study

BACKGROUND: A few population-based studies assessing the etiology of community-acquired pneumonia in both hospitalized and ambulatory patients, with special emphasis on the etiologic role of viral infections, have been reported. The purpose of this study was to assess microbiological differences according to initial site of care in patients with community-acquired pneumonia. METHODS: We studied 496 patients > 14 years of age collected from the study samples of three population-based studies carried out in the same geographical area ("Maresme" region in the Mediterranean coast in Barcelona, Spain) with the same methodology over an 8-year period (1987-1995). RESULTS: Fifty-six percent of patients were hospitalized and 44% were treated at home. Of the 474 patients with etiological evaluation, 195 patients had an identifiable etiology (overall diagnostic yield 41%). Streptococcus pneumoniae was the most common causative organism. Viral infection was diagnosed in 26.5% of hospitalized patients vs. 13.2% of ambulatory patients (P=0.03). Twenty-five percent of the 68 patients with documented etiology treated at home had Chlamydia pneumoniae infection compared with 14.3% of those treated in the hospital. Ten percent of hospitalized patients had pneumonia caused by two pathogens compared with 9.7% of ambulatory patients. The association of viruses and bacteria was the most frequent cause of dual infection (79% inpatients, 67% outpatients). CONCLUSIONS: This study has provided information on etiology of community-acquired pneumonia in hospitalized patients and in patients treated at home. A considerable proportion of patients had viral pneumonia, frequently requiring hospital admission for inpatient care.     

Atypical pathogens in adult patients admitted with community-acquired pneumonia in Korea

This study examined the prevalence of atypical pathogens causing community-acquired pneumonia (CAP) in Korea. We collected sera and clinical data for a period of 1 year for the adult patients consecutively admitted to Chunchon Sacred Heart Hospital with CAP. The diagnosis was made using serologic methods to detect antibodies for Mycoplasma pneumoniae, Chlamydia pneumoniae, Legionella spp., Chlamydia psittaci, and Coxiella burnetii. Among 81 recruited patients, C. pneumoniae (n = 10, 12.3%) was the leading cause of illness, followed by M. pneumoniae (n = 7, 8.6%). One case of C. burnetii pneumonia was detected, but there were no cases of Legionella spp. or C. psittaci. Three cases of C. pneumoniae pneumonia were co-infected with either M. pneumoniae or C. burnetii. There was no significant difference between atypical pneumonia and non-diagnosed pneumonia in terms of clinical manifestations. In conclusion, of the atypical pathogens causing CAP, C. pneumoniae and M. pneumoniae appear to be the important etiologic pathogens in Korea.     

Microbial etiology of community-acquired pneumonia in the adult population of 4 municipalities in eastern Finland.

To determine the etiology of community-acquired pneumonia in the adult population of a defined area, specific antibody responses in paired serum samples, levels of circulating pneumococcal immune complexes in serum samples, and pneumococcal antigen in urine were measured. Samples (304 paired serum samples and 300 acute urine samples) were obtained from 345 patients > or =15 years old with community-acquired, radiologically confirmed pneumonia, which comprised all cases in the population of 4 municipalities in eastern Finland during 1 year. Specific infecting organisms were identified in 183 patients (including 49 with mixed infection), as follows: Streptococcus pneumoniae, 125 patients; Haemophilus influenzae, 12; Moraxella catarrhalis, 8; chlamydiae, 37 (of which, Chlamydia pneumoniae, 30); Mycoplasma pneumoniae, 30; and virus species, 27. The proportion of patients with pneumococcal infections increased and of those with Mycoplasma infections decreased with age, but for each age group, the etiologic profile was similar among inpatients and among outpatients. S. pneumoniae was the most important etiologic agent. The annual incidence of pneumococcal pneumonia per 1000 inhabitants aged > or =60 years was 8.0.     

Mycoplasma pneumoniae and Chlamydia pneumoniae infections in children with pneumonia. Mowgli Study Group.

The most common clinical signs, host responses and radiographic patterns were studied in 203 Italian children hospitalized for community-acquired pneumonia in order to clarify the role of clinical and radiological characteristics in the diagnosis of Mycoplasma pneumoniae and/or Chlamydia pneumoniae infections. Antibody measurements in paired sera and polymerase chain reaction on nasopharyngeal aspirates were used to establish the diagnoses of acute M. pneumoniae and C. pneumoniae infection, and the aetiologic data were correlated with the clinical, laboratory and radiographic data obtained on admission. No significant association was observed between evidence of M. pneumoniae and/or C. pneumoniae infection and periods of episode during the year, mean age of the study subjects, individual symptoms, physical findings or laboratory test results. Furthermore, no significant correlation was observed in relation to the radiological findings and M. pneumoniae and/or C. pneumoniae infection. This study shows that neither clinical findings nor laboratory parameters distinguished Mycoplasma pneumoniae and/or Chlamydia pneumoniae infection in children with pneumonia. Radiological findings also have a limited capacity to differentiate aetiologic agents. The priorities for future research include the development of rapid, easily accessible and cost-effective diagnostic tests useful for each episode of pneumonia in children.     

The etiology of community-acquired pneumonia at an urban public hospital: influence of human immunodeficiency virus infection and initial severity of illness.

In a prospective study, the etiology of community-acquired pneumonia (CAP) was investigated among consecutive patients admitted to an academic, urban public hospital in Seattle. The study population was uniquely young, was predominantly male, and had high rates of homelessness, cigarette smoking, alcoholism, injection drug use, and human immunodeficiency virus (HIV) infection. Leading causes of CAP among HIV-negative patients were aspiration, followed by Streptococcus pneumoniae, Legionella species, and Mycoplasma pneumoniae. Among HIV-positive patients, Pneumocystis carinii, Mycobacterium tuberculosis, S. pneumoniae, and M. pneumoniae were the most common etiologic agents. Severe CAP was associated with typical bacterial infections and aspiration pneumonia but not Legionella infection among HIV-negative patients and with Pseudomonas aeruginosa infections among HIV-positive patients. These findings emphasize the need to tailor empirical antibiotic therapy according to local patient populations and individual risk factors and highlight the importance of recognizing underlying HIV infection in patients who are hospitalized with CAP.     

Evaluation of the polymerase chain reaction method for detection of Streptococcus pneumoniae DNA in pleural fluid samples

STUDY OBJECTIVE: Streptococcus pneumoniae is the most frequent causative agent of community-acquired pneumonia (CAP); however, an etiologic diagnosis by traditional techniques can be accomplished in only a small percentage of patients with CAP. Pleural fluid is present in approximately 40% of patients with CAP; therefore, we hypothesized that detection of S pneumoniae DNA in pleural fluid by polymerase chain reaction (PCR) may help to increase the rate of diagnosis of pneumococcal pneumonia. DESIGN: A prospective study of cases. SETTING: A university hospital in Lleida, Spain. PATIENTS AND METHODS: One hundred two samples of pleural fluid (51 samples from consecutive adult patients with pneumonia and 51 samples from unselected control subjects) were tested by the nested-PCR method to detect selected pneumolysin gene of S pneumoniae, and the results were compared with those provided by alternative diagnostic methods. RESULTS: PCR in pleural fluid had a diagnostic sensitivity of 78% in patients with pneumococcal pneumonia, with positive results in 2 of 2 patients (100%) and 5 of 7 patients (71%) who had positive or negative pleural fluid culture findings, respectively. PCR results were also positive in 3 of 24 patients (12%) with pneumonia of unknown etiology and negative in all patients with pneumonia due to microorganisms other than S pneumoniae. Thus, the calculated specificity was 93%. Among control subjects, PCR gave positive results in two cases (4%). CONCLUSION: The nested-PCR test, applied to pleural fluid samples from patients with CAP, showed a sensitivity of 78% and a specificity of 93% in the diagnosis of pneumococcal pneumonia.     

Early switch from intravenous to oral antibiotics in hospitalized patients with bacteremic community-acquired Streptococcus pneumoniae pneumonia

BACKGROUND: The identification of Streptococcus pneumoniae bacteremia in hospitalized patients with community-acquired pneumonia is considered by some investigators to be an exclusion criterion for early switch from intravenous to oral therapy. OBJECTIVE: To determine whether the switch from intravenous to oral therapy in such patients, once the bx;1patient reaches clinical stability, is associated with poor clinical outcome. METHODS: The medical records of 400 patients with community-acquired pneumonia hospitalized at the Veterans Affairs Medical Center of Louisville (Louisville, Ky) were reviewed to identify patients with bacteremic S pneumoniae. Four criteria were used to define when a patient reached clinical stability and should be considered a candidate for switch therapy: (1) cough and shortness of breath are improving, (2) patient is afebrile for at least 8 hours, (3) white blood cell count is normalizing, and (4) oral intake and gastrointestinal tract absorption are adequate. RESULTS: A total of 36 bacteremic patients were identified. No clinical failures occurred in 18 patients who reached clinical stability and were switched to oral therapy or in 7 patients who reached clinical stability and continued intravenous therapy. Clinical failures (5 deaths) occurred in the group of 11 patients who did not reach clinical stability. CONCLUSION: Once a hospitalized patient with community-acquired pneumonia reaches clinical stability, it is safe to switch from intravenous to oral antibiotics even if bacteremia caused by S pneumoniae was initially documented.     

Nonsevere community-acquired pneumonia: correlation between cause and severity or comorbidity

BACKGROUND: Community-acquired pneumonia frequently constitutes a nonsevere infection manageable at home. However, for these low-risk episodes, the epidemiological features have not been carefully analyzed. OBJECTIVES: To determine the cause of nonsevere community-acquired pneumonia and to investigate if a correlation exists between cause and severity or comorbidity. METHODS: During a 3-year period, all patients with nonsevere community-acquired pneumonia, according to the Pneumonia Patient Outcome Research Team prognostic classification (patients in groups 1-3), were included in the study. Causes were investigated through the following procedures: cultures of blood, sputum, and pleural fluid; serologic tests; and polymerase chain reaction methods to detect Streptococcus pneumoniae DNA in whole blood or Mycoplasma pneumoniae and Chlamydia pneumoniae DNA in throat swab specimens. RESULTS: Of 317 initially included patients, 247 were eligible for the study. A microbial diagnosis was obtained in 162 patients (66%), and the main pathogens detected were S pneumoniae (69 patients [28%]), M pneumoniae (40 patients [16%]), and C pneumoniae (28 patients [11%]). For the 58 patients in prognostic group 1, M pneumoniae was the most prevalent cause, and atypical microorganisms constituted 40 (69%) of the isolated agents. In contrast, for patients in prognostic groups 2 and 3, S pneumoniae was the leading agent, and a significant reduction of M pneumoniae cases and a greater presence of other more uncommon pathogens were observed. The existence of comorbid conditions was not a determining factor for particular causes. CONCLUSIONS: Among low-risk patients with community-acquired pneumonia, there was a certain correlation between severity and cause. In contrast, the existence of a comorbidity did not have a predictive causative value.