C-Reac ive Protein: A New Rapid Assay for M naging Infectious Disease in Primary Health Care

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1 Scand J Prim Health Care 1991; 9: 3-10 C-Reac ive Protein: A New Rapid Assay for M naging Infectious Disease in Primary Health Care PER HJORTDAHL', SVERRE LANDAAS', PETTER URDAL', MARTIN STEINBAKK3, PER FUGLERUD4 and BAARD NYGAARD' 'Department of General Practice, University of Oslo, Frederik Stangs gate 11-13, Oslo, 2Central Laboratory, 3Microbiological Laboratory, Ullevdl University Hospital, Oslo, 'Statistical R&D, Nycomed AS, Oslo, 'Clinical Department, Nycomed Pharma AS, Oslo Hjortdahl P, Landaas S, Urdal P, Steinbakk M, Fuglerud P, Nygaard B. C-reactive protein: A new rapid assay for managing infectious disease in primary health care. Scand J Prim Health Care 1991; 9: Quantitative C-reactive protein (CRP) measurement has become increasingly valuable as a test for rapid diagnosis of infections in hospital medicine. CRP has not obtained the same importance in primary health care. This has, at least partly, been due to methodological difficulties, with no simple or rapid tests with quantitative results available. A new immunometric semi-quantitative assay, NycoCardrM CRP, has recently been developed. CRP was analysed at the local health centres by the new assay in 288 consultations where patients came because of infections. Parallel CRP values were obtained by an established reference method. The two procedures had an acceptable correlation (r = 0.8). The primary care doctors also registered the clinical information they obtained from each CRP result. CRP was helpful in indicating the presence, or absence of bacterial infection in more than half the consultations due to new infections. CRP was thought to yield more clinical information than the erythrocyte sedimentation rate in almost every case. Key words: C-reactive protein, erythrocyte sedimentation rate, immunological tests, near-topatient testing, rapid testing, decentralized testing, clinical usefulness of a test. Per Hjortdahl, MD, Department of General Practice, University of Oslo, Frederik Stangs gate 11-13, N-0264 Oslo 2, Norway. Infectious diseases are among the most common reasons for consultation in primary health care. Diagnosis is usually based on the clinical picture, but support from laboratory examinations is frequently needed. Infectious disease parameters that are routinely used in near-to-patient testing in primary care include the erythrocyte sedimentation rate (ESR), leucocyte count, urinary analyses, and, lately, specific immunological tests. In hospitals, quantitative C-reactive protein (CRP) measurement has become increasingly important as an emergency test for rapid diagnosis of serious infectious disease, such as sepsis, meningitis, and pneumonia (I, 2). It is also used to differentiate between upper and lower urinary tract infections and to monitor the effect of antibacterial treatment (3, 4). CRP has largely replaced the use of the ESR in many clinical settings, especially in departments of paediatrics. CRP has not obtained the same importance in primary health care. This is partly due to the low prevalence of serious infections in general practice. Recently, however, CRP has also been suggested as a useful test in the differentiation between bacteria and viruses in common diagnostic problems such as respiratory tract infections (, 6). This makes the test increasingly relevant for primary care. The limited use of CRP in decentralized units also reflects methodological difficulties, since no simple or rapid tests with quantitative results have been available. Latex agglutination CRP slide tests have been in use for some years. These tests are of limited practical value, however, because they primarily yield qual- I' Scand I Prim Health Care 1991: 9

2 4 P. Hjortdahl et al. itative yedno answers that indicate whether CRP is present above a given concentration or not. A new immunometric semi-quantitative assay for testing CRP concentrations has been developed. This test requires little technical equipment, is rapid to perform, and is relatively inexpensive. The aim of the present study was to evaluate the accuracy of this immunometric CRP test when used in a primary health care setting. Furthermore we wanted to investigate how useful general practitioners found the test in their diagnosis and management of infectious diseases. MATERIALS AND METHODS The six-week study took place at ten communitybased primary health care centres in Oslo during the autumn of The health centres are continuityof-care based general practice units, handling most of the local population s need for primary medical care. Thirty-six doctors entered their patients in the study and evaluated the test results, while 20 auxiliary staff, mainly office nurses, performed the tests. A short introductory session was held at each centre before the study. Patients were included if the doctor entertained a clinical diagnosis of infectious disease, found a medical indication for a venepuncture, and was given informed consent by the patient. 288 consultations were registered, of which 12 were return visits within the registration period. The 276 patients were between 1 and 86 years of age, mean age 38. New infections accounted for 208 of the encounters, while the remaining 80 were follow-up visits. The sedimentation rate was measured at 286 consultations. In their efforts to obtain an aetiological diagnosis the doctors performed 309 additional tests, sent 11 specimens for microbiological culture, and 179 samples for serological testing. Six patients had chest X-rays taken. An aetiological diagnosis was established in 3 of the 20 encounters (17%) with a new infectious disease, wh%r!o causative organisms were found in the remaining 83%. The diagnostic classification was done in accordance with the International Classification of Health Problems in Primary Care (ICHPPC-2 Defined) (7). The CRP was measured at the health centres by a newly developed immunometric assay (NycoCardTM CRP, Nycomed Pharma AS, Oslo). The test is based on CRP-specific antibodies linked to a filtration membrane. When the diluted serum sample is ap- plied to the test card, CRP molecules are trapped to the membrane. A solution containing CRP-specific monoclonal antibodies conjugated with ultra small gold particles is then applied. CRP trapped on the membrane will bind the antibody-gold particle in a sandwich-type reaction. In the presence of pathological levels of CRP, the membrane appears reddishpurple with a colour intensity proportional to the CRP concentration. The colour intensity is evaluated using a reference colour chart enclosed in the test kit. The CRP result was recorded in one of seven categories (< 10,lO-19,20-39,40-9,60-99, , and > 200 mg/l). The laboratory personnel also estimated the exact CRP concentration ( best guess CRP value), based on the colour intensity of the test result. Parallel serum samples were sent to the Central Laboratory at Ullevll University Hospital, where the CRP was analysed by an established turbidimetric assay (reagents from Orion Diagnostics, Finland, applied to a Cobas Bio centrifugal analyser). The serum samples were subsequently frozen (-20 C) and retested 1-3 months later, both with the standard method and with the NycoCard CRP. These last tests were done by one experienced technician who did not know the previous findings. The doctors were to indicate what clinical information, if any, they gained from each CRP result. The usefulness of the test was also evaluated by asking the doctors to what extent they felt each result contributed to the diagnosis or management of the patient s problem. The answer was given as a check-mark on a ten centimetre visual analogue scale (VAS), ranging from no impact to very great impact. The same question was asked about the usefulness of the ESR. Analysis of variance and linear regression analysis were used in the statistical evaluation. The study was approved by the regional Committee on Ethics. RESULTS The test procedure Linear regression analysis of the 194 available best guess values read at the doctors laboratories (y) versus the corresponding values from the Central Laboratory (x) gave a coefficient of correlation r = 0.8 (y = 0.82~ + 6.2) (Fig. 1). When one technician retested the 244 available frozen serum samples using the NycoCard CRP, the correlation with Scand J Prim Healrh Care 1991; 9

3 C-reactive protein Fig. 1. Correlation between the 194 available best guess values read at the health centres and the corresponding CRP values found by the reference method. The diagonal line represents an absolute agreement between the two methods. CRP (mg/l) decentralized 00 the values found at the Central Laboratory (second reading, thaw samples) was r = 0.9 (y = 0.78~ ). The categorical results obtained by NycoCard CRP at the health centres were the same as the 0 CRP (rngfl) by reference method reference method in 138 of the 277 cases where corresponding data were available, and within plus/ minus one category in 268 of these cases (Table I): When the CRP values of the 208 patients who presented with new infections were subdivided into Table I. The results of the imrnunometric CRP assay read at the primary care physician s laboratory, compared with the values found by the reference method. Comparable data available in 277 of the 288 consultations. CRP (mg/l) by reference method < > 200 CRP (mg/l) decentralized < > Total Total Scond J Prim Health Core 1991, 9

4 6 I? Hjortdahl et al. Table IIa. The immunometric CRP results in patients with new infectious disease (n = 208) subdivided into four clinically important groups, compared with the values found by the reference method. ~~~ CRP (mga) by reference method < L 100 Total CRP (mg/l) decentralized < z Total Table IIb. Clinically useful groups when using CRP serum levels as a diagnostic tool with infectious diseases in primary health care (based on references 1, 2,, 9). < 20 mg/l mg/l mg/l mg/l Normal or slightly elevated level Concentrations of CRP in serum between 0 and 10 mg/l are considered normal. Between 10 and 19 mg/l indicates a slight increase, but is of uncertain diagnostic value. Increased level These values are often associated with viral infections. They may be difficult to interpret and use in the diagnostic process. Significantly increased level This increase is usually due to bacterial infections, though some viral infections may be seen in the lower end of this range. The CRP in viral meningitis does not usually rise above mga. Streptococcal infections frequently attain this level. Very high level Almost always caused by bacterial infections; viral infections very rarely seen at this level. four relevant clinical groups (< 20,20-39,40-99, mg/l), the results from the two methods were the same in 182 patients and within plushinus one category in all but two cases (Table 11, a and b). Clinical usefulness The clinical information gained from the CRP results is shown in Table 111. The CRF' was considered helpful in indicating the presence or absence of infection in 60% of the visits due to new infections. It Scand J Prim Health Care 1991; 9 was particularly useful in indicating the absence of bacterial genital infections (10 of 14 cases). CRP was thought to be of help in differentiating between viral and bacterial aetiology in 4% of the 174 new cases with respiratory tract infection. The test was helpful in judging disease activity or in monitoring effect of treatment in 8% of the follow-up consultations. The CRF' was felt to yield little or no relevant clinical information in 11% of the visits. The usefulness of CRP and ESR as related to infectious disease is shown in Figure 2. Among the 77 ESR results above 20 mm, 4% were given a VAS score of five or higher, while 23% of the 208 results below 20 mm obtained five or higher. Among the 7 CRP results above 20 mg/l, 74% scored five or higher, while 42% of the 21 results below this level scored five or above. The value of the tests in different clinical situations is given in Table IV. This table also indicates the usefulness of the tests in new consultations when they are used as diagnostic tools, and in follow-up encounters where they are used mainly to monitor disease activity. DISCUSSION Patients were included in this study if there was a clinical indication for venepuncture to obtain blood for an ESR or other tests. This causes a patient selection towards those with more severe or clinically difficult problems. During a six-week period the participating doctors attend to about 2,00 patients with infectious disease (8). Blood tests were requested in 276, or roughly 10%. This reflects the fact that most infections seen in general practice are minor or self-limiting, with no need for laboratory tests. Previous studies have shown that the majority of infections are seen in the younger age groups (8). In spite of this, the mean age of the patients in our study was 38, suggesting that the younger patients often have uncomplicated infections not requiring blood tests. The general practitioners may also be reluctant to venepuncture children. Accuracy The worse correlation in testing by the less trained personnel (r = 0.8), compared with the experienced technician (r = 0.9), was due partly to interperson variation, with 20 different persons doing the test and interpreting the colour intensity, and partly to intra-personal variations related to new test procedures.

5 C-reactive protein 7 Table 111. Clinical information gained by the CRP results. Results are given in per cent of number of consultations with new infections (n = 208) and follow-up consultations (n = 68). Data were available for 276 of the 288 consultations. New infections FOllOW-up All n = 208 n = 68 n = 276 n (Yo) n (70 ) n ( /.I Absence of bacterial infection 6 (31) 19 (28) 84 (30) Presence of bacterial infection 61 (29) 12 (18) 73 (26) Level of disease activity 86 (41) 27 (40) 113 (41) Effect of treatment 1 (< 1) 12 (18) 13 () Little or no information 18 (7) Other (2) 30-2D- 11 (16) 29 (11) 6 (9) 11 (4) Total categories checked When evaluating the CRP results semi-quantitatively the decentralized findings were correct in 0% of the cases, whereas 97% were within plushinus one category (Table I). The discrepancies occurred especially at lower CRP levels. Among the 119 cases found by the reference method to be between 10 and 19 mg/l, 79 (66%) were read immunometrically as being below 10 mg/l. In this range of CRP concentrations differences in colour intensities are slight and may be difficult to interpret. Furthermore, the turbidimetric reference method is, in itself, not very precise at such low concentrations of CRP. Per cent of consultations 1 For diagnostic purposes in primary care, CRP is most valuable in differentiating viral from bacterial infections. It appears that mg/l CRP is a relevant differential range, since most viral infections are below or within this interval, while bacterial infections usually are above (1, 2,, 9). Grouping the CRP values into the four relevant clinical categories (Table IIb), there was agreement in 88% of the cases, and 99% were within plushinus one category (Table IIa). Using 40 mg/l as a threshold value the two procedures agreed in 9% of the 277 tests evaluated. This shows that the irnmunometric test 10 - n- T Fig. 2. The usefulness of the ESR and the CRP results as indicated on the visual No Very great analogue scales. n-esr = impact Scores (in centimeters) impact 286, n-crp = 282. on visual analogue scales Scand I Prim Health Care 1991; 9

6 ~~ 8 P. Hjortdahl et al. Table IV. The usefulness of the ESR and CRP results in different clinical settings. Scores were indicated on a visual analogue scale, range 0-10; 0 = no impact 10 = very great impact. Scores are given as means and standard deviations (S.D.). ESR values were available in 286 and CRP in 282 of the 288 consultations (one patient was not classifiable as to new or follow-up consultation). ESR CRP n Mean S.D. n Mean S.D. New consultations Upper respiratory tract nonspecific sinusitis tonsillitis Lower respiratory tract Urinary tract Genital tract Other Follow up consultations Upper respiratory tract nonspecific sinusitis tonsillitis Lower respiratory tract Urinary tract Genital tract Other All consultations yields acceptable results in nine out of ten clinical situations related to new infectious diseases in primary care. In spite of special efforts by the participating doctors, aetiological diagnoses were obtained in only 17% of the cases. Due to this low level of diagnostic specificity, which is common in family medicine (lo), the present material is not suited for the calculation of CRP test characteristics such as sensitivity and specificity. Clinical usefulness The ten health centres that participated in this study were selected on the basis of practical considerations, and not because the doctors or laboratory personnel were especially enthusiastic about trying the new test procedure. In spite of this, a certain anticipation bias might have been present when the participating doctors subjectively evaluated the clinical usefulness of the test. There are also inherent difficulties in evaluating new test principles of which one has limited theoretical knowledge and little or no practical experience. The results should therefore be interpreted with due caution Patients were included in the study if the doctor thought they might have an infectious disease. In general practice, with its low prevalence of serious disease, one of the important functions of a nonspecific test like CRP is to rule out disease. In the present study, 7% of the patients with new infections had CRP values below 20 mg/l. In one-third of the cases with new infections the doctors found the CRP helpful in ruling out bacterial aetiology. Since CRP was thought to be valuable in the differentiation between viral and bacterial infection in 60% of all new cases, it may become an important diagnostic adjuvant in the differential diagnosis of throat infections, pneumonia, and genital tract infections (, 6, 11). Only % of the registrations were due to urinary tract infections. Most urinary tract infections seen in primary care are cases of cystitis not warranting blood tests. Furthermore, upper urinary tract infections, for which it has been suggested that CRP is useful (3), are often seen at house-calls, where few laboratory tests are available. CRP was useful in monitoring disease activity or Scand I Prim Health Care 1991; 9

7 C-reactive protein 9. treatment response in more than half of all the follow-up patients. CRP versw ESR A new test procedure should yield more information than procedures currently in use, or have practical advantages over them. ESR is one of the most commonly used laboratory tests in general practice, and physicians have extensive experience in interpreting the results (12). A raised ESR is often seen in patients with infectious disease. Serious bacterial infections usually result in high ESR values, but the discrimination against viral infections seems less distinct with the ESR than with CRP. The sedimentation rate increases relatively slowly and may remain elevated for weeks after the infection has subsided (13). ESR increases during pregnancy, making it less suitable as a disease parameter during this period (14). It is easy to perform, but the one hour waiting period often results in the patient leaving the office before the result is available. The CRP concentration usually increases more than a hundred times within 24 hours after the onset of serious infection (1). The half-life of CRP is short and the level falls rather quickly after effective therapy or spontaneous improvement of disease (1). Pregnancy does not cause significant changes in CRP levels (16). The immunometric test procedure evaluated required serum. The procedure itself took less than two minutes, but required more hands-on time and was somewhat more cumbersome to do than the ESR. The time-consuming step of producing serum has recently been eliminated, however, because the test can now be done using whole blood (personal communication, Nycomed Pharma AS). In the present study the doctors indicated equal or better information from the CRP than the ESR in 96% of the consultations. The distribution of the usefulness of ESR was skewed toward the less impact end of the visual analogue scale, while the value of CRP was more evenly distributed along the whole scale (Fig. 2). When dealing with new infections, the physicians found CRP more useful than ESR in all seven clinical diagnostic categories (Table IV). The same trend was seen in the follow-up consultations, with only one exception: the group others including problems such as gastrointestinal and skin infections. Here ESR was considered more relevant than the CRP. The greatest perceived difference in usefulness between ESR and CRP was in the groups of new patients presenting with tonsillitis or sinusitis, and among follow-up patients with genital tract infections. In conclusion, CRP appears to have advantages as an infectious disease parameter in primary care, and to yield more information of clinical value than the ESR. The new CRP assay is fairly accurate and easy to perform in the decentralized laboratory. Further studies are needed, however, to evaluate test characteristics such as sensitivity, specificity, and likelihood ratio with respect to the different infections seen in primary care. This should preferably be done using whole blood samples. To adjust for the expectation bias, a re-evaluation of the usefulness of the test should be done as the novelty of the procedure vanishes. ACKNOWLEDGEMENTS We want to thank the participating doctors and laboratory personnel at the following health centres in Oslo: Hovseter, Klemetsrud, Lysejordet, Majorstua, Rustad, R@a, Stovner, T@yen, Vinderen REFERENCES Lindback S, Hellgren U, Julander J, Hansson L. The value of C-reactive protein as a marker of bacterial infection in patients with septicaemia, endocarditis and influenza. Scand J Infect Dis 1989; 21: Morley J, Kushner I. Serum C-reactive protein levels in disease. Ann NY Acad Sci 1982; 389: Hellerstein S, Duggan E, Welchert E, Mansour F. Serum C-reactive protein and the site of urinary tract infections. J Paediatrics 1982; 100: 21-. Schofield K, Voulgari F, Gozzard D, Leyland M, Beeching N, Stuart J. C-reactive protein concentration as a guide to antibiotic therapy in acute leukemia. J Clin Pathol 1982; 3: Whicher J, Chambers R, Higginson J, Nashef L, Higgins P. Acute phase response of serum amyloid A protein and C-reactive protein to the common cold and influenza. J Clin Pathol 198; 38: Melbye H, Straume B, Aasebe U, Brox J. The diagnosis of adult pneumonia in general practice. Scand J Prim Health Care 1988; 6: ICHPPC-2 Defined. International Classification of Health Problems in Primary Care. Oxford: Oxford University Press, Rutle 0. An analysis of patient encounters in general practice.. Oslo: The National Institute of Public Health - Unit for Health Services Research, In Norwegian, English summary. Scand J Prim Health Care 1991: 9

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