Chest Radiograph

The chest radiograph is the most useful test to establish the diagnosis of community-acquired pneumonia. The American Thoracic Society and the Infectious Diseases Society of America guidelines recommend a chest x-ray in all patients suspected of pneumonia.^1,16

 A chest x-ray helps predict severity of disease, as multilobar infiltrates and pleural effusions are associated with increased mortality.²⁴A chest x-ray is useful to rule out other conditions that mimic pneumonia.

Although a chest x-ray cannot determine the etiology of a community-acquired pneumonia, certain classic radiographic findings may suggest a specific etiology, such as the diffuse bilateral infiltrates of Pneumocystis carinii pneumonia or the upper-lobe cavitary lesions of Mycobacterium tuberculosis. A cavity with an air-fluid level is essentially diagnostic of a lung abscess caused by anaerobes.

Limitations Of Chest Radiography

Regrettably, the chest x-ray is not a perfect screening tool, and patients may have a normal radiograph in the face of pneumonia.¹⁹ Some authorities believe this is more likely to occur in patients with dehydration and those with profound neutropenia. However, the scientific evidence for this is meager.

Despite all that is written about classic radiographic features of pneumonias caused by various bacterial pathogens, radiographic findings are nonspecific.¹⁷ Finding an infiltrate on the chest x-ray does not ensure the diagnosis of pneumonia. False-positive chest x-ray findings may be due to unilateral pulmonary edema, neoplasms, atelectasis, hypersensitivity pneumonitis, pulmonary embolism, and sarcoidosis. Even radiologists cannot reliably distinguish bacterial from viral pneumonias on chest x-ray.²⁰More importantly, it is also impossible to distinguish pulmonary embolism from pneumonia on chest film.¹⁹

There is considerable interobserver variability in the roentgenographic diagnosis of pneumonia—a variability that does not improve with increasing medical experience. In one study comparing physicians of differing experience, agreement with a “gold standard” (3 radiologists) was poor. Dense lobar or segmental opacities were uniformly recognized as pneumonia, while patchy opacities and diffuse alveolar disease were frequently missed as signs of pneumonia, or misinterpreted.²⁵

Who Needs A Chest X-ray?

Since the minority of patients who present to the ED with a chief complaint of cough will have pneumonia, ordering chest x-rays for all patients with respiratory symptoms is wasteful.¹⁰Factors that predict pneumonia on chest x-ray are temperature greater than 37.8°C, heart rate greater than 100 beats/min, rales, locally decreased breath sounds, and the absence of asthma.¹¹Dementia is also associated with radiographic pneumonia. The absence of any abnormality in the patient’s temperature, pulse, or respiratory rate significantly reduces the odds of a positive film.¹

Laboratory Testing

Many patients with community-acquired pneumonia need no diagnostic tests beyond a history and physical  xamination, chest x-ray, and pulse oximetry. Today, physicians are ordering fewer tests to determine the etiology of pneumonia, especially in those well enough to be treated at home. In  ne recent study, only 29.7% of outpatients had any microbiologic tests performed, and in only 5.7% was a discrete organism found.²⁶Despite (or because of) the paucity of tests, these patients had excellent outcomes.

Extensive testing often adds expense and little benefit. Hospitalized patients who undergo extensive diagnostic testing (including blood gas analysis, blood, sputum, CSF, urine cultures, and bronchoscopy) have no better  outcomes than those who undergo minimal diagnostic procedures.²⁷Controlling for severity of illness, an aggressive diagnostic strategy merely adds costs and increases length of stay without improving care.

When using the PORT scoring algorithm, patients in risk class I need no laboratory testing (except possibly pulse oximetry). However, others may benefit from laboratory testing to determine the safety of outpatient management. The PORT study found the following laboratory variables to be important in predicting mortality:

pH < 7.35

BUN > 10.7 mmol/L (30 mg/dL)

Sodium < 130 mEq/L

Glucose > 13.9 mmol/L (250 mg/dL)

Hematocrit < 30%

pO₂ < 60 mmHg

The PORT algorithm suggests these tests to determine risk stratification for patients who have significant risk factors on history and physical examination. Still, the study never tested the hypothesis that selective testing of these laboratory parameters based on clinical judgment is safe and effective.

Measurements Of Oxygenation

Hypoxia is associated with increased morbidity and mortality in patients with community-acquired pneumonia. Measuring the patient’s oxygen saturation using pulse oximetry may greatly influence medical decisionmaking. In one study, the failure to adequately assess oxygenation was responsible for 22% of preventable deaths associated with pneumonia.²⁸ The PORT authors suggested that admitting all patients with an oxygen saturation of less than 90% on room air would provide an additional margin of safety to their decision algorithm.²

An arterial blood gas (ABG) is helpful in certain patients with low oxygen saturation. This includes patients hospitalized with underlying pulmonary disease, such as COPD, to evaluate for possible CO₂ retention. Patients with an altered mental status may demonstrate CO₂ retention or an acid-base disorder on blood gas analysis. The ABG is also helpful in patients suspected of having PCP, because the decision to add steroids to the antibiotic regimen is based on the PaO₂ being less than 70 mmHg or an A-a gradient of 35 or greater.

While the pO₂ and pCO₂ are important parameters, the base deficit may be equally or even more important in the evaluation of a patient with community-acquired pneumonia. A significantly negative base deficit provides an early warning of occult shock. This metabolic acidosis may portend a more complicated hospital course consistent with sepsis, intoxication, or a metabolic disorder.

Microbiologic Testing

There is no need to routinely search for a pathogen in the patient with community-acquired pneumonia who is well enough to be treated at home. However, the need for microbiologic testing in the patient who requires  ospitalization remains controversial. Possible microbiology tests include sputum tests (Gram’s stain, culture, antigen testing, and polymerase chain reaction [PCR] tests), blood assays blood cultures, antigen tests, and serology), and urine testing (cultures and antigens). Transtracheal aspiration and bronchoscopic sputum samples are selective alternatives to expectorated sputum. These techniques are impractical for routine use due to their invasive nature, the need for technical expertise, and expense. Indications are limited to patients with fulminant disease or disease unresponsive to empiric antimicrobial therapy. There is no need for an emergency physician ever to perform a  diagnostic transtracheal aspiration.

One prospective study (n=122) examined the value of blood cultures, sputum analysis, and serologic tests in patients with community-acquired pneumonia. A definitive etiologic agent was identified in only 26% of patients.  Because the results of these tests affected antibiotic therapy in only 8% of patients, the authors concluded that  these tests should not be routine; rather, they should be ordered based on severity of illness.⁴Other studies reinforce the precept that routine microbial investigation, such as sputum investigation, blood cultures, and bronchoscopy, provide little value for patients hospitalized for communityacquired pneumonia.²⁹

While the literature on the need for microbiologic testing is mixed, the physician must keep a central caveat in mind: Diagnostic testing should never delay antibiotic treatment of the acutely ill patient. The severely ill patient requires early antibiotics, not extensive culturing. In such cases, rapidly obtain blood cultures and administer antibiotics. Even the IDSA supports this position in their clinical guideline.¹

Sputum Gram’s Stain

Gram’s stains are unnecessary in the patient who will be treated at home. For the patient ill enough to require hospitalization, the ATS and IDSA disagree on the role of the sputum Gram’s stain. The ATS does not recommend routine Gram’s staining of sputum, citing the lack of correlation with sputum cultures and the fact that mortality is unchanged whether or not the etiology is established.⁷

On the other hand, the IDSA guidelines state that for hospitalized patients with community-acquired pneumonia, establishing the etiologic agent offers a more precise and possibly more cost-effective use of antibiotics. The IDSA recommends that sputum be obtained from a deep cough before antibiotic treatment and that it be rapidly transported and processed (within 2 hours). They agree that antibiotics should not be delayed for specimen collection in the patient who is acutely ill. Adequate samples should contain less than 25 squamous epithelial cells per low power field.¹ Direct staining of sputum may be useful to diagnose pulmonary infections caused by Mycobacterium tuberculosis and endemic fungi.

Limitations Of Sputum Gram’s Stains

The disadvantages of the Gram’s stain are related to poor sensitivity and specificity. If the Gram’s stain is inaccurate, the patient may be treated with the wrong antibiotic. The ATS believes that the empiric treatment of likely pathogens is more effective than basing antibiotic choices on this unreliable test.

Gram’s staining is an art (some say a lost art) that depends upon the skill of the person performing and interpreting the stain. In one study, Gram’s stain interpretation by house staff showed a 43% false-positive rate for  Streptococcus pneumoniae and a 73% false-negative rate for Haemophilus influenzae.³⁰

The results of sputum Gram’s stains and sputum cultures often conflict. The positive predictive values for common Gram’s stain morphotypes and their corresponding cultures vary tremendously, from 7.1% to 90.6%.³¹In several studies, Gram’s stain could not reliably predict the presence of gram-negative rods, including Haemophilus influenzae.^32,33Gram’s stains are also unable to identify intracellular pathogens, such as Legionella sp., Mycoplasma pneumoniae, and Chlamydia pneumoniae.

While obtaining a sputum specimen poses little risk to the patient (unless a transtracheal aspirate is attempted), induction of a sputum sample poses a real danger to everyone in the ED—staff, patients, and visitors. If the patient is infected with TB, sputum induction with saline irritants will aerosolize the tuberculous bacilli. While some curmudgeons suggest that nebulized sputums be obtained in the administrative offices of HMOs, a more appropriate venue is a negative pressure room.

Sputum Cultures

Numerous studies indicate that sputum cultures are neither sensitive nor specific. Contamination with oral microbes and previous antibiotic therapy further limits their utility. Prior antibiotic therapy is associated with false-positive cultures for Staphylococcus aureus and gramnegative bacilli.³⁴

Sputum cultures may be helpful in certain cases. Sputum cultures with antibiotic sensitivity testing may be useful in patients who have failed prior antibiotics, and perhaps in those suspected of having a penicillinresistant Streptococcus pneumoniae. Cultures are also important in the diagnosis of Mycobacterium tuberculosis and endemic fungi. Polymerase chain reaction (PCR) technology applied to sputum may detect Mycobacterium tuberculosis in a matter of hours; however, it is not routinely available.

Blood Cultures

Drawn for a variety of febrile conditions, blood cultures taken in the ED rarely yield positive results. Overall, only 1.6% of blood cultures taken in the ED impact patient care.³⁵ They are a “Pavlovian” response to fever, with a true positive rate averaging 20% and a falsepositive rate of about 50%.³⁵ Neither the ATS nor the IDSA recommends blood cultures in patients who do not require hospitalization.

However, both the ATS and IDSA recommend blood cultures for all hospitalized patients. Blood should be cultured from two separate sites before the administration of antibiotics.

Limitations Of Blood Cultures

The gold standard for diagnosis of community-acquired pneumonia is recovery of an organism from blood culture. Yet only 11% of hospitalized patients with community-acquired pneumonia have positive blood cultures.²¹ The detection of a specific pathogen does not improve the mortality rate in hospitalized patients with community-acquired pneumonia.^4,22

Blood cultures rarely change management of hospitalized patients with pneumonia. In one study, positive blood cultures were obtained in 6.6% of hospitalized patients with pneumonia; in 25 other patients (4.8%), positive  cultures were felt to be contaminated. A positive blood culture led to therapeutic alteration in just over 1% of patients who were cultured. The authors concluded that the clinical utility of blood cultures is limited and the cost-effectiveness is questionable.³⁶Other studies support this position.^37,38

The role of blood cultures in the treatment of antibiotic-resistant organisms is unclear. The incidence of penicillin-resistant Streptococcus pneumoniae has steadily increased over the past decade, and current community rates vary from 20-40% in the United States. However, the in vitro antibiotic resistance may not correlate with the clinical efficacy of an antibiotic (see the section on antibiotics later in this article).

Other Tests

Surely a CBC should be drawn in patients with pneumonia, right? Interestingly, the white count was not associated with mortality in the PORT study.⁶ While the hemoglobin or hematocrit is of concern, the leukocyte count should not be used in decision making (except possibly in patients undergoing cancer chemotherapy).

It is the chest film, not routine blood tests, that provides the diagnosis of community-acquired pneumonia. Severity of illness, usually apparent on clinical examination, may correlate with abnormal laboratory values. Hyponatremia occurs more frequently in community-acquired pneumonia due to Legionella sp. than other pathogens.²⁵

The IDSA recommends routine HIV screening for the hospitalized patient if the prevalence of HIV infection is moderate within a community.²⁴ In general, the emergency physician should avoid testing for HIV. These tests are governed by strict regulations. Some states demand formal pretest and post-test counseling—requirements that are difficult to meet in the ED.

In 1993, the American Thoracic Society recommended that serologic tests not be routine, due to their low yield in community-acquired pneumonia. Acute and convalescent titers are usually necessary for interpretation. On occasion, a urinary antigen assay for Legionella pneumophilia is helpful, as it is both rapid and reasonably sensitive (70%) for this disease.  However, a negative result does not exclude the diagnosis.³⁹

Thoracentesis

 A significant number of patients with pneumonia will develop a pleural effusion. These parapneumonic effusions are associated with increased morbidity and mortality. Both the ATS and IDSA recommend thoracentesis in patients with pneumonia and a significant pleural effusion. While the definition of “significant” may vary, some authorities suggest thoracentesis for effusions that measure more than 10 mm on a lateral decubitus film.⁴⁰Either the emergency medicine or admitting physician may perform thoracentesis, which can be accomplished hours after antibiotic administration.

The most important laboratory parameters of pleural fluid are pH, LDH, and glucose measurement, as these discriminate between complicated vs. uncomplicated effusions.⁴¹Patients with purulent fluid or those with a pH less than 7.2 require chest tube drainage. Other tests include white blood cell count and differential, Gram’s stain, acid-fast stain, and bacterial, fungal, and mycobacterial cultures.

 

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