We have presented the first estimates of the incidence of RA according to age and sex in a Danish population. Although it is conceivable that almost every incident case had been ascertained at the hospital, there is an inherent risk that some patients may have been treated in general practice solely or by doctors outside the county. However, in the Danish health care system every citizen is entitled to specialist care free of charge and data from the public Health Insurance suggested that almost every RA patient in the region had been ascertained at the hospital. Consequently, we do not think that the number of patients treated by doctors outside the hospital could have been high.
The present study was retrospective and when using classification criteria the ascertainment of cases depends on the quality of data in the medical records. At the hospital 19 patients were diagnosed with RA primarily by rheumatologists who often did not make detailed notes. If they had, some of the diagnosed patients might have been discovered to have fulfilled the 1987 ACR criteria.
Central characteristics of the cases in our study did not change over the study period and this indicates that the diagnostic threshold among the doctors at the hospital did not change either. However, the possibility that we have included patients with undifferentiated arthritis as cases needs to be addressed.
In our study we included as cases some patients in whom the time criterion related to morning stiffness and joint swelling was not formally fulfilled. This approach is in line with a study from the UK where the patients were classified as having RA after one examination only [7
]. The patients included in our study had symptoms associated with RA for at least six weeks prior to being classified as having RA. It is conceivable that these patients had also joint swellings in that period and, consequently, we do not think that this approach lead to the inclusion of patients with undifferentiated arthritis.
Moreover, Wolfe, et al
studied 638 patients with undifferentiated arthritis and 503 patients with RA. In the two groups 12% and 81% were RF positive, respectively. After two years of follow-up, the disease had resolved in 54% in the group with undifferentiated arthritis and in the RA group 8% were in remission [26
]. In the present study, the proportion of RF positive patients was 76%. We would therefore expect our patients to have a prognosis similar to that which was described for the RA patients in the study by Wolfe, et al
At our hospital, it has previously been documented that the majority of the patients referred to the EAC did not have joint swellings and only 7% of the patients had undifferentiated arthritis [22
]. The fraction of patients with undifferentiated arthritis was lower than what has been reported from two other EACs [27
]. It therefore seems unlikely that the introduction of the EAC in the study period may have lead to the incorrect inclusion of a substantial number of patients with undifferentiated arthritis in our study.
The increasing trend observed in our study may have been an artefact caused by changes in referral patterns or induced by the relatively short study period.
The EAC introduced at our hospital in 1998 received patients with recent onset arthritis. The clinic may have increased the probability that patients with RA were ascertained at the hospital. Moreover, in a study of RA patients from the UK, a decrease in the time from the first symptom to hospital referral has been described [29
]. The UK study indicates a growing awareness among general practitioners of the importance of early diagnosis and treatment of RA. In our study, both the introduction of the EAC and changing medical practice among general practitioners could explain the increasing trend in the number of cases ascertained over the study period.
On the other hand, in a study from Rochester, Minnesota, a cyclic pattern in the annual incidence rates was observed from 1955 to 1995. Irrespective of the fact that overall the incidence of RA decreased over the period, time intervals of different length with either increasing or decreasing rates were observed [3
]. It could be that our data, which were collected over a relatively short period of time, reflected a time interval with increasing rates in an overall pattern of decreasing rates.
The mean annual incidence described in the present study was slightly higher than what has previously been reported in studies from other Scandinavian countries, the UK, and North America, using the 1987 ACR criteria. In the following section the incidence refers to rates per 100 000 PY.
In a study from the UK using prospective notification of patients with inflammatory polyarthritis, the incidence of RA was 36 in females and 14 in males. The estimates were based on one examination only, but if the results of another examination 12 months later had also been included, the incidence was higher [7
In Oslo, Norway, cases were identified retrospectively from a hospital register. The mean annual incidence from 1988 to 1993 was 37 in females and 14 in males [12
]. In another hospital study from Northern Norway, the incidence was 36 in females and 21 in males [11
]. In the Norwegian studies, it was not described whether a systematic search for misclassified cases had been performed in the registers. In other studies where register data have been used, up to 11% of the RA cases had been misclassified [1
In a study from Rochester, Minnesota, the incidence was 33 in females and 26 in males from 1985 to 1995 [3
]. In that setting, RA cases were identified using data from hospital records, nursing homes and private practising physicians serving the population. A similar approach was used in a study from Finland, where the incidence was 46 in females and 25 in males [8
]. On the basis of data from a drug reimbursement register in Finland, the incidence of RA was 43 in females and 24 in males in 1995 [9
]. In our study, the rates were close to the ones reported from Finland where data were collected from several sources including primary health care. In our opinion, this makes it plausible that almost every patient with incident RA were included in the present study.
In Denmark, the occurrence of RA has previously been investigated in a study from general practice from 1983. Using the 1958 American Rheumatism Association criteria for definite RA, the number of new patients with RA over a 13-week period was divided by the number of patients who attended the participating practices [31
]. The reported proportion of patients with incident RA was 24/100 000 persons, but this measure is difficult to interpret and compare with the results of our study. It is therefore not possible to discern whether the incidence of RA in Denmark has changed over the last decades.
In our data, we observed a decrease in the rates for both male and females older than 74 years. A decrease in rates in the oldest age groups has been observed in different studies [3
] and, as previously noted by Symmons, et al
], the reason for this could be that RA in older females is under-diagnosed or taken for osteoarthritis.