The Diagnosis of Adult Pneumonia in General Practice

Transcription

1 Scand J Prim Health Care 1988; 6: The Diagnosis of Adult Pneumonia in General Practice The Diagnostic Value of History, Physical Examination and Some Blood Tests HASSE MELBYE,'. * BJ@RN STRAUME,' ULF AASEBo3 and JAN BROX4 'General Practice, Troms), 'Institute of Community Medicine, University of Troms), 'Depurtment of Chest Medicine and 'Department of Clinical Chemistry, University Hospital of Tromse, Troms), Norway Melbye H, Straume B, Aaseb~ U, Brox J. The diagnosis of adult pneumonia in general practice. Scand J Prim Health Care 1988; 6: Because of lower respiratory infection that was treated with antibiotics on the suspicion of pneumonia, 71 patients aged 15 years or more were referred to the study by general practitioners. Using a positive chest X-ray as a "gold standard", 15 % had pneumonia. The diagnostic value of variables from history, physical examination and blood tests was evaluated by calculating the likelihood ratio (LR). A duration of illness less than 24 hours before consulting the general practitioner was the variable from the history with the highest LR, The white blood cell count and particularly the C-reactive protein analysis had a high diagnostic value, CFtP >SO mgh had an LR of 37. In thii selected material pulmonary symptoms and lung findings were of minor value in differentiating patients with and without pneumonia, with no LR exceeding 2.3. This can be explained to some extent by selection bias. Key words: pneumonia, crackles, CRP-analysis, diagnostic value, likelihood ratio. Hasse Melbye, Institute of Community Medicine, University of 'koms0, PB 417, N-9001 'Ikoms0, Norway. In Norway, the diagnosis of pneumonia in general practice is usually based on clinical findings. Sometimes it is supported by blood tests, but a chest X- ray is rarely available. The difficulties in diagnosing pneumonia, as defined by a positive chest X-ray, merely by clinical evaluation, has been shown by Diehr et al. (1) and by Leventhal (2). A striking discrepancy between stethoscopic and radiographic findings in pneumonia was found by Osmer & Cole (3). The predictive value of blood tests in the diagnosis of pneumonia has been studied in paediatric patients, and C-reactive protein has been shown to be a very promising test in a study by McCarthy et al. (4). In order to improve the care of patients with lower respiratory infection, our objective was to evaluate the value of different clinical and laboratory variables in,the diagnosis of pneumonia. MATERIAL AND METHODS The study included 71 patients, aged 15 years or more, who were treated with antibiotics by a general practitioner for a suspected pneumonia. The di- agnoses were made without the aid of chest X-rays. Excluded were patients who were admitted to hospital and those who were too ill to attend the Department of Chest Medicine's Outpatient Clinic one to four days after the start of antibiotic therapy. All the 25 general practitioners in Troms@ (a town with 5OOOO inhabitants), as well as the doctors in the primary health care emergency ward were asked to refer patients to the study. The study took place during a four-month period, from February to May The general practitioners recorded the results of the physical examination on a standard form that listed the examinations to be carried out. The diagnostic procedures were not further standardised. The two symptoms first presented by the patients as the reason for encounter were recorded on the form. The general practitioners were also asked to estimate the probability of pneumonia using a three level scale, more than 0.75, between 0.75 and 0.25 and below 0.25, and to report the two findings judged to be most indicative of pneumonia. The patients attended the Department of Chest Scund J Prim Health Cure 1988: 6

2 112 H. Melbye et al. Table I. Clinical findings reported by general practitioners to be most indicative of pneumonia in 71 patients treated with antibiotics for suspected pneumonia Two fmdings were listed on each patient No. Typical history (including cough, fever, dyspnoea, chest pain) 52 Crackles 31 Other lung findings 22 Long duration of disease 10 A history of frequent pneumonia 4 Miscellaneous (including fatigue and general signs of disease) 11 Per cent Medicine s Outpatient Clinic one to four days later. The symptoms associated with the present illness and its duration were recorded on a questionnaire. Blood was taken for erythrocyte sedimentation rate (ESR), white blood cell count (WBC), peripheral blood-smear, and C-reactive protein-analysis (CRP). Chest X-rays were taken with postero-antenor and lateral projections. They were interpreted independently by two doctors (at least one senior) at the Chest Department. When the first X-ray was interpreted by one or both doctors as having a density indicative of pneumonia, a new X-ray was taken after approximately four weeks. Densities that showed signs of resolution on the follow-up X- ray were used as evidence of pneumonia and as a gold standard in the analysis of the diagnostic values. (In two cases that lacked follow-up X-ray, an obvious pneumonic infiltrate was deemed sufficient evidence of pneumonia.) The diagnostic value of the clinical variables, or the ability to differentiate pneumonia patients (PN) from non-pneumonia patients (NPN), were evaluated by calculating the sensitivity, specificity, positive predictive value (PPV), and likelihood ratio (LR) of each variable. The LR of a test is the ratio between the probability that a person with a disease has a positive test (the sensitivity) and the probability that a person without the same disease has a positive test (1- specifcity). Thus, LR = sensitivity/( 1 -specificity). According to Bayes theorem the LR is related to the pre-test odds and the posttest odds in the following way: pre-test odds x 15 Table 11. Diagnostic value of symptoms presented as reason for encounter by 71 patients treated with antibiotics for a suspected pneumonia in general practice One or two symptoms were recorded on each patient Likeli- Positive Sensi- Speci- hood predictive Symptom tivity ficity ratio value Cough Fever Chest pain Dyspnoea Fatigue Fear of having pneumonia LR = post-test odds. Using probabilities instead of odds, the PPV can be calculated as follows: prevalence x LR PPV = 1-prevalence+prevalencexLR. The LR, unlike the PPV, is not directly dependent on the prevalence of the disease, but is susceptible to selection bias (5). Statistical significance was tested by two-tailed chi-square test with Yates correction. Table 111. Diagnostic value of duration of illness before consulting the general practitioner and symptoms reported on a questionnaire by 71 patients treated with antibiotics for suspected pneumonia in general practice I11 <1 day Ill <4 days Ill >7 days coryza Sore throat Headache Cough Yellow sputum Bloody sputum Dyspnoea Chest pain Fever chills Liketi- Positive Sensi- Speci- hood predictive tivity ficity ratio value Scand J Prim Health Care 1988; 6

3 Diagnosis of adult pneumonia in general practice 113 Table IV. Diagnostic value of physical findings in 71 patients treated with antibiotics for suspected pneumonia in general practice Likelihood Positive Findings Sensitivity Specificity ratio predictive value Sick appearance Cough attack when inspiring deeply Respiratory rate >24/min Crackles One lung Both lungs Wheezes One lung o 0.15 Both lungs Both lungs without crackles Decreased breath sounds o 0.15 Dullness to percussion o 0.15 Pleural rubs These findings reported on only 64 and 65 patients respectively. RESULTS Eleven or 15% of the 71 referred patients had pneumonia, according to our radiologic gold standard. The mean age in the pneumonia group was 44 years. Six were males and five were females. In the total material the mean age was 48 years, ranging from 15 to 79 years, 37 males and 34 females. The patients were referred from 36 doctors; 28 were referred from general practitioners surgeries, 29 from the emergency ward, and only three from home visits. Table I lists the clinical findings reported by the general practitioners to be most indicative of pneumonia in the referred patients. Typical symptoms such as high fever, cough, dyspnoea and chest pain were considered important in 73% and crackles in 44% of the patients. The diagnostic value of the variables, regarded as pneumonia-tests, are shown in Tables 11-VI. None of the reasons for encounter had an LR exceeding 1.7 (Table 11). Cough and fever were presented by most of the patients in both groups, while dyspnoea was only presented as reason for encounter by NPN patients. Pneumonia patients generally had a shorter duration of illness before consulting the general practitioner than the NPN patients, and the LR of a duration less than 24 hours was 13.5 (Table 111). Three of the PN patients, but only one NPN patient, had been ill for this short time, and the finding was of statistical significance (p<o.os). None of the PN patients had been ill for more than I1 days, in contrast to 14 of the NPN patients. The common symptoms of upper and lower respiratory infection had all low diagnostic values. Coryza and sore throat were found in about half of the patients in both groups. Cough, fever, dyspnoea, chest pain (unspecified), headache and chills were the symptoms most frequently encountered in the PN patients, but these symptoms were also common in the NPN patients. None of the physical findings had an LR that exceeded 2.3 (Table IV). Most of the patients in both groups had a sick appearance. Crackles were found in seven and wheezes in four of the PN patients. In the NPN group these lung sounds had about the same frequency, found in about half of the patients. Wheezes over both lungs were more strongly associated with the NPN group than wheezes over only one lung, especially when there were no crackles. The more infrequent findings, pleural rubs, decreased breath sounds, and dullness to percussion, also had low diagnostic values. The LR of tachypnoea, with a respiratory rate of 24/min or more, was only 1.8. An ESR of 20 mm/h or more was found in ten of the PN patients and in 34 of the NPN patients, giving a specificity of only 0.43 (Table V). The PPV and LR increased when the level was set to 50 mm/h, but the sensitivity of the test decreased radically. A WBC count over 9.5X109 was found in about Scand J Prim Health Care 1988; 6

4 114 H. Melbye et al. Table V. Diagnostic value of blood tests in 69 patients treated with antibiotics for suspected pneumonia in general practice Likelihood Positive Sensitivity Specificity ratio predictive value. ESR >20 d ESR >50 mm/h WBC >9Sx 109 WBC >ll.5xlo9 WBC <6.5~109 Blood smear diff. counts Lymphocytes >SO% Neutrophils 65% Stabs >lo% C-reactive protein CRP >11 mgll CRP >50 mga Blood smear was only examined in 58 patients of whom ten had pneumonia. half of the PN patients and gave an LR of 5.0. A WBC count below 6.5~10 was found in only one PN patient, but in nearly half of the NPN patients. A lymphocytosis (on peripheral blood smear) of more than 50% lymphocytes was only encountered in the NPN group. All but one of the patients with lymphocytosis also had a low WBC count. The finding of more than 10% stabs had an LR of 3.2. It was encountered in six of ten PN patients and in nine of 48 NPN patients. CRP analysis was the best discriminator between the PN and the NPN patients. Eight of the eleven PN patients had a CRP of 50 mg/l or more, opposed to only two of 57 NPN patients (P<O.OOl). Thus CRP >50 mg/l had an LR of 37.0 and a PPV of 0.8. The general practitioners regarded six patients as having a very high probability of pneumonia (Table VI). Three of these had pneumonia. Pneumonia was less frequent in the medium-probability group than among the patients regarded as having the lowest probability of pneumonia DISCUSSION The prevalence of pneumonia of only 15% in our material may seem remarkably low. However, this result is not exceptional. In the study by Diehr et al. the prevalence of pneumonia among patients with acute cough in whom the doctors sought chest X-ray in the clinical evaluation, was only 6 % (1). In Leventhal s study the prevalence of pneumonia was 19% (2). Defining pneumonia by a positive chest X-ray is generally accepted in the medical literature. Minor pathological changes may lack a radiographic counterpart (6), and some invisible pneumonias have been reported (7, 8), but according to Goodman et al. this is uncommon (9). According to WONCA s ICHPPC-2-Defined pneumonia may be defined by clinical findings alone by three of the following clinical signs: 1) diminished air entry, 2) dullness to percussion, 3) bronchial breath sounds, 4) fine crepitations, and 5) Table VI. Diagnostic value of estimates of probability for pneumonia made by general practitioners in 71 patients treated with antibiotics for suspected pneumonia Probability Likelihood Positive Sensitivity Specificity ratio predictive value Higher than 75 % Between 25 % and 75 % Lower than 25 % Scand J Prim Health Care 1988: 6

5 Diagnosis of adult pneumonia in general practice 115 increased vocal fremitus and resonance. Such abundant findings were rarely found in our study, and many pneurnonias would probably be missed using this clinical definition (3). In a recent study by Woodhead et al., pneumonia is defined as an acute lower respiratory tract infection, for which antibiotics were prescribed, associated with new (i.e. previously unrecorded) focal signs on exarnination of the chest (10). This definition does not exclude acute bronchitis. It is rather an inclusion criterion than a definition of a disease, but it reflects a common view of pneumonia among general practitioners. An important problem in relying on chest X-rays in the diagnosis of pneumonia, is the interobserver variability of interpretation (1 1). However, when control-films of the patients are available, this insecurity is probably strongly reduced. Taking the X-ray a few days after the start of treatment with antibiotics is probably of little significance. According to a hospital study (12), less than five per cent of the pneumonias show signs of resolution on the X-rays after five days of antibacterial treatment. Using our radiographic gold standard of pneumonia in the calculation of diagnostic values, the insufficiency of the clinical examination in the diagnosis of pneumonia in patients who are not seriously ill was confmned. The general practitioners put great emphasis on the history in their diagnosis. In our study the variables from the history had low diagnostic values, except the information of a short duration of illness. Some general practitioners reported a long duration of disease to be indicative of pneumonia, but in our study none of the PN patients had been ill for more than two weeks. This result, and results from studies that show bronchial hyperreactivity after viral respiratory infections (13), may indicate that patients with a chest infection of more than two weeks, without dramatic changes in illness shortly before the consultation, probably have bronchitis in almost every case. However, mycoplasma pneumonia or psittacosis should be considered, especially in children and young adults. The LR of cough was higher when presented as a reason for encounter than when recorded at history-taking. This probably reflects that the PN patients had a more troublesome cough. The opposite was the case for the LR of dyspnoea. The reason for this seems to be that serious dyspnoea is more frequently a symptom of obstructive lung disease than of moderate pneumonia. The low diagnostic value of crackles in our study contrasts with the emphasis put on this finding by the general practitioners. Fry et al. (14) also maintain the view that crackles are a cardinal sign of serious chest infections that need antibiotic treatment, and they proposed chest infections with crackles as a clinical entity in general practice. Laennec, the inventor of the stethoscope, found crackles to be a typical sign of pneumonia, when he compared clinical and autopsy findings (15). Nowadays, ambulatory PN patients constitute a quite different selection of patients, and crackles are frequently not audible. The low specificity of crackles may be explained by their occurrence in lung disorders such as fibrosis, interstitial oedema, and bronchitishronchiolitis, which has been explained by pathophysiological studies (16). Wheezes in pneumonia are probably caused by the co-existence of bronchitis in most instances. Pleural rubs were found relatively frequently in our NPN-patients. This was unexpected, and it is hard to give a good explanation. Artefacts, e.g. rubbing between the stethoscope and the skin or between the scapula and thoracic wall, are possible explanations. Local dullness to percussion and decreased respiratory sound may be hard to recognize, unless there is a considerable infiltrate; they are probably findings with a particularly low interobserver consistency. Leventhal found that tachypnoea was the best clinical indicator of pneumonia in children (2), with an LR that can be calculated to be 4.2. In our study, a respiratory rate of 241min or more had an LR of 1.8. This discrepancy may be explained by the greater difficulties in evaluating the respiratory rate in adults. Unless the patients have dyspnoea, the respiration is often more or less invisible and hard to count. In our study the general practitioners were asked to count the inspirations while pretending to auscultate the heart, asking the patient to breathe naturally. The voluntary influence on the respiration was not eliminated by this method, and anxiety provoked by the consultation probably also tended to raise the respiratory rate. These errors may partly be neutralized by a judgement of the patient s behaviour. A critical assessment of the respiratory rate is probably a useful test of pneumonia, even in adults. The diagnostic values of the blood tests were Scand J Prim Health Care 1988: 6

6 116 H. Melbye et al. generally higher than of the clinical variables. In judging these results, the delay between the start of antibiotics and the blood-sampling must be considered. All the tests are affected by this delay of one to four days. The ESR normally rises even after the start of effective treatment, while WBC and CRP values start to fall immediately and are often normalized after one week (17). The LR of the WBC-count was unchanged, and the test became much more sensitive, when lower- ing the upper normal limit from 11.5~ lo9, which is the reference value at our laboratory, to 9.5~10~. The delay before blood sampling is probably the reason for this. The LR of the CRP test was outstanding in our study, when the diagnostic level was 50 mga. Blood-sampling at the consultation with the general practitioner would result in generally higher CRP values, and further studies must be done to establish the most practical level in the diagnosis of pneumonia. The CRP test was carried out by a method not in use in general practice. Rapid slide tests, however, are commercially available. These tests require different dilutions of the patient s sera to estimate the concentration of the CRP. Clinical trials should be carried out to evaluate the usefulness of these slide tests in general practice. Compared to the study by Diehr et al. (l), the typical symptoms and signs of pneumonia had a higher sensitivity and a lower specificity in our study. Two types of selection bias may explain this: 1. Bias associated with the case-mix or the general seventy of illnesddisease in the material. 2. Bias associated with the emphasis put on certain findings by the general practitioners. The inclusion criterion in the study by Diehr et al. was acute cough. The majority of the pneumonias were not recognized by the clinicians, and PN patients with a mild disease were probably well represented in that study. In our study, the moderately ill are certainly overrepresented, having more symptoms and signs than those with a mild disease. In the study by Diehr et al., the sensitivities of crackles and chills were 0.19 and respectively, while in our study they were 0.64 and The first type of selection bias may partly explain these differences. The sensitivities of crackles were certainly also influenced by the second type of bias, since one of the two findings most indicative of pneumonia in Z3 of the patients in whom they were found. The generally lower specificities of the clinical variables in our study are probably also caused by the relative underrepresentation of PN and NPN patients with mild disease (bias type one). A similar decrease in specificity has also been observed in other studies (18). The specificity of crackles was probably still more influenced by the other type of bias, namely the high status of this finding among the general practitioners. The selection biases reduce the specificities in our study more than they raise the sensitivities. Thus, the likelihood ratios are generally lower in our study than in the study by Diehr et al. (e.g. an LR of crackles of 1.2 compared to 2.7). In other words, the clinical variables seem to be more useful in selecting patients from a case-mix with a low prevalence of pneumonia for further investigation, than they are in diagnosing pneumonia in the selected group. It is accepted that pneumonias should be treated with antibiotics, as bacterial aetiology is frequent (19). The treatment of acute bronchitis is more controversial (14), but the results of clinical trials indicate that antibiotics should probably be withheld in most cases (20). In order to select patients with a lower respiratory infection for antibacterial treatment, the CRP analysis may prove to be a valuable test (4), and the usefulness of this test in general practice needs further evaluation. ACKNOWLEDGEMENTS We thank the participating general practitioners and chest physicians, the staff-members at the Emergency Ward, the hospital laboratories, and the University Library. Special thanks to Carina Koflaath, Vigdis Nzss and Helen Schei at the Chest Department s Outpatient Clinic. The work was supported by LHL (a national association for heart and lung disease) and the University Hospital of Tromsgi. REFERENCES 1. Diehr p, wood RW, Bushyhead J, Krueger L, wolcott B, ~ ~ RK. Bediction ~ of ~ pneumonia f in i ~ ~ outpatients with acute cough: A statistical approach. J Chronic Dis 1984; 37: Leventhal JM. Clinical predictors of pneumonia as a guide to ordering chest roentgenogram. Clin Pediatr 1982; 21:73~. 3. Osmer JC, Cole BK. The stethoscope and roentgenothe general practitioners reported crackles to be gram in acute pneumonia. South Med J 1966; Scand J Prim Health Care 1988; 6

7 Diagnosis of adult pneumonia in general practice McCarthy PL, Frank AL, Ablow RC, Masters SJ, Dolan TF. Value of C-reactive protein test in the differentiation of bacterial and viral pneumonia. J Pediatr 1978; 92: Radack KL, Rouan G, Hedges J. The likelihood ratio. Arch Pathol Lab Med 1986; 110: Conte P, Heitzman ER, Markarian B. Viral pneumonia. Roentgen pathological correlations. Radiology 1970; 95: Stein MT. Delayed roentgenographic signs associated with acute pneumonia in children. J Fam Pract 1981; 12: 63W. 8. Aderka A, Sidi Y, Garfinkel D, Rothem A, Weinberger A, Pinkhas J. Roentgenologically invisible mucormycosis pneumonia. Respiration 1983; 44: Goodman LR, Goren RA, Teplick SK. The radiographic evaluation of pulm onary infection. Med Clin North Am 1980; 64: Woodhead MA, MacFarlane JT, McCracken JS, Rose DH, Finch RG. Prospective study of the aetiology and outcome of pneumonia in the community. Lancet 1987; 1: Stickler GB, Hoffman AD, Taylor WF. Problems in the clinical and roentgenographic diagnosis of pneumonia in young children. Clin Pediatr 1984; 23: Weitzman S, Heimer D, Naggan L, Bar-Ziv G, Glick S. Uncomplicated pneumonia: an evaluation of determinants of length of hospitalization. Isr J Med Sci 1983; 19: Hall WJ, Douglas RG. Pulmonary function during and after common respiratory infections. Annu Rev Med 1980; 31: Fry J, White R, Whitfield M. Respiratory disorders. Library of general practice. Churchill Livingstone, Jiennec RTH. Treatise on the diseases of the chest. 4th ed. London: Longman, Forgacs P. Lung sounds.-landon: Bailliere Tindall, Hanson LH, Wadsworth Ch. C-Reactive protein and its diagnostic usefulness-especially in infections. Med Lab 1980; 8: Rozanski A, Diamond GA, Bennan D, Forrester JS, Moms D, Swan HJC. The declining specificity of exercise radionuclide ventriculography. N EngI J Med 1983; 309: Macfarlane JT, Finch RG, Ward MJ, Macrae AD. Hospital study of adult community-acquired pneumonia. Lancet 1982; 2: Williamson H. Acute bronchitis. A homely prototype. for primary care research. J Fam Pract 1986; 23: Manuscript received March 20, 1987, revised manuscript November 20, 1987, accepted November 22, 1987 Scand J Prim Health Care 1988; 6