Objectives To quantify the diagnostic accuracy of selected inflammatory markers in addition to symptoms and signs for predicting pneumonia and to derive a diagnostic tool.
Design Diagnostic study performed between 2007 and 2010. Participants had their history taken, underwent physical examination and measurement of C reactive protein (CRP) and procalcitonin in venous blood on the day they first consulted, and underwent chest radiography within seven days.
Setting Primary care centres in 12 European countries.
Participants Adults presenting with acute cough.
Main outcome measures Pneumonia as determined by radiologists, who were blind to all other information when they judged chest radiographs.
Results Of 3106 eligible patients, 286 were excluded because of missing or inadequate chest radiographs, leaving 2820 patients (mean age 50, 40% men) of whom 140 (5%) had pneumonia. Re-assessment of a subset of 1675 chest radiographs showed agreement in 94% (κ 0.45, 95% confidence interval 0.36 to 0.54). Six published “symptoms and signs models” varied in their discrimination (area under receiver operating characteristics curve (ROC) ranged from 0.55 (95% confidence interval 0.50 to 0.61) to 0.71 (0.66 to 0.76)). The optimal combination of clinical prediction items derived from our patients included absence of runny nose and presence of breathlessness, crackles and diminished breath sounds on auscultation, tachycardia, and fever, with an ROC area of 0.70 (0.65 to 0.75). Addition of CRP at the optimal cut off of >30 mg/L increased the ROC area to 0.77 (0.73 to 0.81) and improved the diagnostic classification (net reclassification improvement 28%). In the 1556 patients classified according to symptoms, signs, and CRP >30 mg/L as “low risk” (<2.5%) for pneumonia, the prevalence of pneumonia was 2%. In the 132 patients classified as “high risk” (>20%), the prevalence of pneumonia was 31%. The positive likelihood ratio of low, intermediate, and high risk for pneumonia was 0.4, 1.2, and 8.6 respectively. Measurement of procalcitonin added no relevant additional diagnostic information. A simplified diagnostic score based on symptoms, signs, and CRP >30 mg/L resulted in proportions of pneumonia of 0.7%, 3.8%, and 18.2% in the low, intermediate, and high risk group respectively.
Conclusions A clinical rule based on symptoms and signs to predict pneumonia in patients presenting to primary care with acute cough performed best in patients with mild or severe clinical presentation. Addition of CRP concentration at the optimal cut off of >30 mg/L improved diagnostic information, but measurement of procalcitonin concentration did not add clinically relevant information in this group.
Pneumonia was diagnosed by chest x radiography in 140 (5%) of the 2820 patients presenting to primary care with acute cough. The optimal combination of symptoms and signs for predicting pneumonia was absence of runny nose and presence of breathlessness, crackles and diminished breath sounds on auscultation, tachycardia, and fever. Signs and symptoms were useful in correctly identifying patients with a “low” (<2.5%) or “high” (>20%) diagnostic risk in 26% of patients. In the 74% of patients in whom diagnostic doubt remained (estimated risk 2.5%-20%), measurement of C reactive protein (CRP) concentration helped to correctly exclude pneumonia. A simplified diagnostic score based on symptoms, signs, and CRP concentration resulted in proportions of pneumonia of 0.7%, 4%, and 18% in the low, intermediate, and high risk group, respectively. Measurement of procalcitonin concentration had no clinically relevant added value in this setting.
Strengths and limitations
This is the first study to quantify the independent diagnostic value of symptoms, signs, and additional diagnostic value of inflammatory markers for pneumonia in patients presenting with acute cough in primary care that included an adequate number of cases of pneumonia. All blood samples were analysed in the same laboratory with standardised procedures. Serum CRP and procalcitonin concentrations were measured by conventional venous blood tests in a diagnostic laboratory and not with a point of care test. The added value of CRP might be different and could be lower when measured with a point of care test in general practice. Nonetheless, agreement between point of care test results and a conventional reference test has been shown to be good.44
Given how common lower respiratory tract infections are, many more eligible patients presented during the recruitment period than were approached about participation in this study, and therefore we probably did not achieve the goals of recruiting all consecutive, eligible patients. Nevertheless, we do not believe that there was important clinical selection bias because feedback from recruiting clinicians during and after the study was that the time required to recruit and assess each patient made sequential recruitment of every eligible patient impossible.
Chest radiographs were examined by local radiologists. We attempted to increase uniformity in assessment by implementing a protocol for reporting. While there was some variability between observers, the moderate unweighted κ of 0.45 was similar to that reported in other studies.18 20
We did not attempt to distinguish between bacterial and viral pneumonia as this is not feasible in routine primary care.14 45 All available relevant guidelines advocate identification of patients with pneumonia and treatment with antibiotics without further aetiological testing.14
Comparison with other studies
Absence of a runny nose and presence of dry cough, breathlessness, chest pain, diarrhoea, fever, and crackles have previously been found to have diagnostic value for pneumonia in primary care populations.7 9 “Tachycardia” and “diminished vesicular breathing” have diagnostic value in secondary care populations.3 6 8 11 We were able to confirm the predictive value of most of these items, apart from chest pain and diarrhoea. Differences between our findings and those from previous studies could relate to the difference in prevalence of pneumonia, inclusion criteria, and outcome definition.
Our finding that CRP concentration can be low in people with pneumonia is not new. Flanders and colleagues reported on a small subgroup of patients with pneumonia who had a CRP of less than 11 mg/L.3 In the 54 patients with pneumonia with low CRP in our study, the estimated diagnostic risk of pneumonia was high (n=3) or intermediate (n=51) based on history and physical examination results as defined in our model. These findings emphasise that CRP test results should be interpreted together with clinical findings.
Of the factors known to lower CRP—such as steroid use46 and duration of disease47—only steroid use (including both oral and inhaled steroids) was significantly more prevalent in the group of patients with pneumonia with low CRP concentration. Exclusion of all steroid users from our analyses resulted in a similar association between CRP concentration and pneumonia.
Procalcitonin concentrations in our study were higher in patients with pneumonia and comparable with previous findings in patients with lower respiratory tract infection in primary care.17 48 They did not, however, add meaningful diagnostic information. Holm and colleagues showed a clear association between procalcitonin concentration and radiographic pneumonia as well as bacterial infection,17 but the positive predictive value was too low to be useful in clinical practice. Our findings support this conclusion. Moreover, Holm and colleagues studied a population with a higher prevalence of pneumonia (13%) and did not combine history and physical examination with procalcitonin test results.17
Implications for practice and conclusions
Although the diagnostic “symptoms and signs” model presented in this study assigned an intermediate diagnostic risk of pneumonia to most patients, history taking and physical examination alone enabled general practitioners to correctly identify a small group of patients at high risk. Chest radiography and/or (empirical) antibiotic treatment should therefore be considered in these patients. In these more severely ill patients, point of care tests, including CRP, do not seem to be useful. In patients with a low risk of pneumonia based on symptoms and signs, it seems justified to withhold further diagnostic investigation and not to treat with antibiotics.
CRP has additional diagnostic value in patients with an intermediate diagnostic risk of pneumonia as determined by symptoms and signs alone, especially in appropriately excluding pneumonia. Procalcitonin has no additional diagnostic value in primary care.
The simplified score derived from the regression models is more suitable for uptake in daily care than the regression models. The downside of the simplified score is that it is less precise and contains less diagnostic information. To determine whether our diagnostic model improves clinical outcomes in everyday practice would require an implementation study in which general practitioners use point of care CRP testing with outcomes such as patient recovery and the unnecessary prescription of antibiotics. Further research should also determine the performance of CRP in other settings where pneumonia is more prevalent or where patients are more severely ill.
What is already known on this topic
- Studies have evaluated the diagnostic accuracy of signs and symptoms for pneumonia, but there is limited evidence applicable to primary care
- The added diagnostic value of C reactive protein (CRP) and procalcitonin concentrations to clinical signs and symptoms is unknown
- Symptoms and signs (absence of runny nose and presence of breathlessness, crackles and diminished breath sounds on auscultation, tachycardia, and fever) have moderate diagnostic accuracy for pneumonia in patients who present in primary care with acute cough
- CRP concentration at the optimal threshold of >30 mg/L adds some diagnostic information by increasing diagnostic certainty in the patients when doubt remains after history and physical examination
- Procalcitonin concentration adds no clinically relevant information in primary care
What this study adds