Is CRP a Useful Predictor of Illness Severity in CAP?
Is CRP a Useful Predictor of Illness Severity in CAP?
Abstract & Commentary
By David J. Pierson, MD, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington. Dr. Pierson reports no financial relationships relevant to this field of study. This article originally appeared in the June 2008 issue of Critical Care Alert. It was edited by David J. Pierson, MD, and peer reviewed by William Thompson, MD. Dr. Thompson is Staff Pulmonologist, VA Medical Center; Associate Professor of Medicine, University of Washington. Dr. Thompson reports no financial relationships relevant to this field of study.
Synopsis: In this examination of blood C-reactive protein levels and clinical data in 570 patients hospitalized with community-acquired pneumonia, initial CRP levels < 100 mg/L were associated with fewer complications, less use of mechanical ventilation and/or inotropes, and lower 30-day mortality. Whether these findings provide useful information beyond what is apparent on routine clinical assessment remains to be seen.
Source: Chalmers JD, et al. Am J Med. 2008: Mar; 121(3):219-225.
For two years, beginning in february, 2005, every patient admitted to the Royal Infirmary of Edinburgh with community-acquired pneumonia (CAP) had blood levels of C-reactive protein (CRP) measured at the time of admission and (if still in the hospital) four days later. A standardized set of clinical and laboratory data were also acquired for each patient, per hospital routine. For this study, Chalmers and colleagues reviewed this data set, along with other information in the patients' hospital charts and their discharge diagnoses, to determine whether a relationship existed between CRP levels and illness severity. Patients were excluded from analysis if they had hospital-acquired pneumonia, cancer, immunosuppression, or chronic pulmonary or liver disease. Associations were sought between CRP levels and the following variables: 30-day mortality, the need for invasive mechanical ventilation and/or inotropic support (not further defined), and "complicated pneumonia" (lung abscess, thoracic empyema, or complicated parapneumonic pleural effusion).
During the period of the study, 936 patients were admitted with CAP, of whom 366 were excluded, leaving 570 for analysis. Median duration of hospitalization was 5 days (interquartile range, 2-11). The number of patients admitted to the ICU is not given, but 13.5% of the patients received mechanical ventilation and/or inotropic support, 7.3% were classified as having complicated pneumonia, and the 30-day mortality rate was 9.6%. By multivariate logistic regression, a low initial CRP value (< 100 mg/L) was associated with a reduced risk of death within 30 days (odds ratio, 0.18, P = 0.03), less need for mechanical ventilation and/or inotropes (OR 0.21, P = 0.02), and a lower incidence of complicated pneumonia (OR 0.05, P = 0.003). Of the 570 patients, 268 had a repeat CRP measurement on hospital day 4; in these patients who remained hospitalized 4 days after admission, a repeat value > 50% of the admission CRP was associated with increased mortality, the need for mechanical ventilation and/or inotropes, and complicated pneumonia (all, P < 0.0001). Chalmers et al conclude that CRP is an independent marker of disease severity among patients hospitalized with CAP.
CRP is an acute phase protein made in the liver, one of a number of markers of inflammation that are detectable in the blood of patients with pneumonia. Its measurement has been available for many years. The hypothesis of this study was that the level of CRP in the blood of a patient hospitalized with CAP, on admission and again after 4 days, could be used to predict illness severity and outcome. Unfortunately, several aspects of the study design seriously impede the meaningful testing of this hypothesis.
The study is described as prospective, and it is in the sense that CRP levels were obtained on all CAP patients beginning on a certain date, and that the hospital's pro-forma documentation of clinical data on all patients continued from that date. However, the indicators of disease severity appear to have been examined retrospectively, using whatever was recorded in the patients' charts by the physicians caring for them rather than specific (prospective) information sought for this study. Why invasive mechanical ventilation and the administration of inotropic agents are combined into a single severity indicator is not explained, and the investigation routine and diagnostic criteria for lung abscess, empyema, and complicated parapneumonic effusion are not described. Most likely, Chalmers et al simply used the diagnoses of these conditions recorded by the physicians caring for the patients; however, these may have been derived. A key question that cannot be answered from the data provided is, "Would measuring CRP levels in a patient with CAP help me to manage that patient, or tell me something about disease severity or prognosis that is not otherwise readily apparent?"
Thinking about the potential clinical value of measuring CRP in patients with CAP, I am reminded of an aphorism attributed to Eugene Stead, Duke University's legendary and long-reigning chief of medicine, about the value of the erythrocyte sedimentation rate (ESR) in determining whether a patient had something seriously wrong with them: "This patient needs either a "sed rate" or a doctor but not both." The point is that the ESR as a general indicator of a systemic inflammatory state tells us nothing beyond what should be evident on a good bedside clinical assessment. The CRP may be another version of the ESR in assessing severity of illness among patients with CRP. Whether this is true cannot be determined from this study, whose results have not convinced me to start measuring CRP in such patients.In this examination of blood C-reactive protein levels and clinical data in 570 patients hospitalized with community-acquired pneumonia, initial CRP levels < 100 mg/L were associated with fewer complications, less use of mechanical ventilation and/or inotropes, and lower 30-day mortality. Whether these findings provide useful information beyond what is apparent on routine clinical assessment remains to be seen.
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