Contemporary epidemiology of gout in the UK general population

Gout is a common and excruciatingly painful inflammatory arthritis affecting at least 1% of the population in Western countries [1, 2]. Gout results from the deposition of urate (monosodium urate monohydrate) crystals in a joint, leading to an acute inflammatory response, or deposition in soft tissues, which can form tophi. Hyperuricemia is considered the precursor of gout, while alcohol intake, obesity, meat and seafood consumption are among the established life-style-related risk factors [3, 4]. Factors reducing renal clearance of uric acid such as use of diuretics, and comorbid conditions such as hypertension, and chronic renal impairment are also involved in the pathogenesis of gout [3]. Pharmacologic interventions remain an important part of gout management [5, 6].

Several studies have suggested an increase in the disease burden of gout [1, 4, 7–10]; however, recent data since the new millennium are lacking so far. The aims of this study were to estimate the contemporary incidence of gout in a UK primary care setting, to quantify the magnitude of associations with potential risk factors, including relevant medical conditions and drug exposures, and to describe recent treatment trends in incident gout patients.

A nested case-control study was conducted within an assembled cohort in the Health Improvement Network (THIN) database. THIN contains computerized information entered by general practitioners in the UK [11]. Data on about 3,000,000 patients are systematically recorded and sent anonymously to THIN. THIN collects and organizes this information in order for it to be used for research projects. The computerized information includes demographics, details from general practitioners' (GPs) visits, diagnoses from specialists' referrals and hospital admissions, results of laboratory tests and a free text section (information available on request). Prescriptions issued by general practitioners are directly generated from the computer. An additional requirement for participating practices is recording of the indication for new courses of therapy. The Read classification is used to code specific diagnoses, and a drug dictionary based on data from the Multilex classification is used to code drugs [12, 13]. The study was approved by a UK Multicentre Research Ethics Committee.

The study population included all individuals aged between 20 to 89 years with a registration status of permanent or died in THIN between January 2000 and December 2007. They were required to have at least two years of enrolment with the GP, and at least one GP visit and one prescription in the two years before entering the source population (start date). This was done to ensure a minimum complete source of information equal to or greater than this time interval prior to entering the study cohort. Individuals with cancer before the start date were excluded. For the ascertainment of incident gout patients, we also excluded all individuals with a gout diagnosis before the start date (considered as prevalent gout) from the study population. All individuals contributed person-time on their respective start date until the earliest of one of the following criteria, whichever came first: code suggesting gout, death, 31 December 2007 or 90^th birthday. Our final study population consisted of 1,775,505 individuals followed up an average of 5.2 years.

We identified 24,768 newly diagnosed gout patients corresponding to a crude incidence rate of 2.68 (95% CI 2.65 to 2.72) per 1,000 person-years among individuals aged 20 years or older. The incidence of gout was 4.42 (95% CI 4.36 to 4.48) in men and 1.32 (95% CI 1.29 to 1.35) in women per 1,000 person-years. The incidence of gout increased with increasing age (Figure 1). The corresponding male/female ratio was 3.4. When we restricted our population to individuals aged 70 years and above the corresponding male/female ratio was 2.3. Mean age at first diagnosis of gout was 60.1 years (95% CI 59.9 to 60.4) among men and 67.7 years (95% CI 67.3 to 68.0) among women, respectively.

Incidence rate was 2.67 (95% CI 2.59 to 2.75) in 2000 to 2001 and 2.52 (95% CI 2.46 to 2.57) in 2006 to 2007. Sex and age specific incidence rate remained relatively constant between both time periods (Figure 2).

Among suspected medical conditions, prior history of congestive heart failure was strongly associated with an increased risk of incident gout, whereas ischemic heart disease was modestly associated with an increased risk. While hypertriglyceridemia was associated with a clear increased risk of gout (multivariate OR, 1.45 (95% CI, 1.18 to 1.79), other dyslipidemias and hypercholesterolemia presented an OR of 1.21 (95% CI, 1.12 to 1.31), and 1.08 (95% CI, 1.02 to 1.14), respectively. No increased risk was found among patients with prior nephrolithiasis, whereas psoriasis was associated with a slightly increased risk of gout (multivariate OR, 1.12; 95% CI, 1.04 to 1.22). The only medical condition associated with a reduced risk of gout was diabetes with an OR of 0.66 (95% CI 0.62 to 0.71) (Table 1).

Among suspected medications, current users of low dose aspirin presented a multivariate OR of 0.95 (95% CI 0.90 to 1.00). When we analyzed the association with aspirin < = 75 mg daily (mini-dose aspirin), which was the dose used in 80% of aspirin users, the estimate of risk remained the same (0.95, 95% CI 0.90 to 1.00). NSAID current users showed a multivariate OR of 3.07 (95% CI 2.90 to 3.24) (Table 2).

When we also analyzed the association according to the time when NSAID treatment was started, it was apparent that the risk was mainly concentrated among current users within the first month with an OR of 7.01 (95% CI 6.46 to 7.61) while those with a use of one year or greater presented an OR of 1.11 (95% CI 1.01 to 1.23), suggesting reverse causality (that is, potential NSAID: treatment for early gout symptoms), among users in the first month of use (data not shown).

Use of cyclosporine was rare, but was associated with an OR of 3.72 (95% CI 2.17 to 6.40). The risk was rather constant over treatment duration with an OR of 4.16 (95% CI 2.21 to 7.83) among users of one year or longer. Azathioprine, another post-organ transplantation drug, showed an OR of 0.93 (95% CI 0.64 to 1.34). We found women on hormone replacement therapy (HRT) to have an OR of 0.86 (95% CI 0.74 to 1.01).

Next, we analyzed gout treatment patterns during recent years. Cumulative incidence of specific gout treatment at different time intervals among the subcohort of newly diagnosed gout patients in 2000 to 2001 (N = 4,349) is presented in Figure 3. One year after first-ever gout diagnosis, use of allopurinol presented the highest cumulative incidence with 28.2% (95% CI, 26.9 to 29.6) followed by colchicine with 13.8% (95% CI 12.8 to 14.8). Use of uricosuric agents was very low with a cumulative incidence of 0.2% (95% CI, 0.1 to 0.4). At nine years, the cumulative incidence of the various gout treatments was about twice as high compared to the use during the first year after gout diagnosis. This pattern was consistent among the three anti-gout treatments.

When we analyzed the use during the first year in a cohort of gout diagnosed in 2006 to 2007 (N = 7,089), the cumulative incidence was 24.3% (95% CI 23.3 to 25.3) for allopurinol and 0.2% (95% CI 0.1 to 0.3) for uricosuric drugs. Only colchicine showed a higher use in 2006 to 2007 with a cumulative incidence of 19.7% (95% CI 18.8 to 20.7) compared to 13.8 (95% CI 12.8 to 14.8) in 2000 and 2001 (Figure 4).

In this large UK primary care population, we found that the incidence of gout remained stable between 2000 and 2007. Our contemporary data corroborated previously shown risk factors, including alcohol intake, obesity, chronic renal impairment, cardiovascular conditions, and diuretic use. Furthermore, we have examined other purported medical conditions and medications, about which we are not aware of any previous large-scale evidence. We found that prior histories of ischemic heart disease, heart failure, hyperlipidemia, and psoriasis are independently associated with an increased risk of gout, whereas diabetes was associated with a lower risk of gout. Interestingly, among subtypes of hyperlipidemia, hypertriglyceridemia was associated with an increased risk, whereas this pattern was much less clear among patients with hypercholesterolemia. We also found that among post-transplant drugs cyclosporine was the only agent associated with increased risk of gout. Finally, the frequency of anti-gout medication use has remained relatively unchanged in recent years except for an increase of colchicine.

Our estimate of incidence rate increased from 0.4 per 1,000 person-years in women of 40 to 49 years, to 3.6 in women of 70 to 79 years. Our results are in line with a prior study [16] that reported an incidence rate of gout of 0.6 per 1,000 person-years in women <45 years and 2.5 in women over the age of 75. When we restricted our cohort to individuals aged 40 years and older, our estimate of incidence (3.5 per 1,000 person-years) was in close agreement with a recent UK study, which estimated an incidence rate of gout 3.2 per 1,000 person-years [9]. Although the mean age of first diagnosis was 60.1 years in males and 67.7 years in females, almost 40% of all incident gout cases were younger than 60 years, indicating that many middle-aged subjects were affected. Male preponderance was more marked in individuals less than 60 years. The different age dependence between males and females in the incidence of gout could in part be due to hormonal status. The uricosuric effects of estrogens could lead to a protective effect on the risk of gout in premenopausal women [17–20]. We also observed that women on exogenous hormonal treatment had 14% less risk of developing gout, and the reduced risk disappeared after stopping HRT, a finding previously reported in a large prospective cohort study of female nurses [16].

Our findings confirm the relationship between traditional risk factors with well-known effects on increasing uric acid levels and subsequently the risk of gout [21, 22]. Our cohort data showed a clear dose-response relation between alcohol consumption and the risk of gout, similar to previous studies [23]. Similarly, our results showed a continuous increased risk of gout with increasing BMI as well as a protective effect in individuals with a BMI under 20 kg/m². Bidirectional associations between weight loss and the prevention of gout and weight gain and the development of gout were reported in the Health Professionals Follow up Study [10]. Chronic renal impairment more than doubled the risk of developing incident gout, which is likely due to decreased urate excretion resulting in uric acid accumulation [4, 24]. Beyond the effect of this and other medical conditions, the use of diuretics, in particular among users of one year and longer than one year, was associated with a three-fold increased risk of gout.

We also evaluated other suspected risk factors lacking in large-scale epidemiologic data, such as lipid abnormalities, ischemic heart disease, congestive heart failure, psoriasis, and various medications. While all subtypes of hyperlipidemia showed independent associations with the risk of incident gout [25], the association with hypertriglyceridemia was most prominent. Elevated triglyceride level is a cardinal feature of insulin resistance, which is closely associated with elevated serum uric acid levels. Individuals with diabetes showed a reduced risk of gout. Some studies have reported lower uric acid levels among diabetes patients [26, 27]. We also found that prior history of congestive heart failure and ischemic heart disease was independently associated with an increased risk of gout. Associated relative tissue hypoxia, increased lactate levels, or accelerated adenosine triphosphate (ATP) consumption could increase the risk of hyperuricemia and gout in patients with these conditions [28, 29]. Recently, a case-control study with only 60 individuals with gout and 6 with heart failure (HF) reported an increased risk of gout among patients with HF [30, 31]. Interestingly, this study also reported that diuretic use did not increase the risk of gout after adjusting for HF and other cardiovascular conditions, as these factors are closely associated with each other [30, 31]. Our study, with substantially larger study samples (n with gout = 24,768 and n with HF = 3,564), was able to detect independent associations with each of these two factors. Psoriasis is a disorder associated with increased cell turnover leading to increased uric acid production [3] and our cohort confirmed a small but significantly increased risk of incident gout. Finally, the absence of an association between history of nephrolithiasis and risk of gout was consistent with the result reported by Kramer et al. [32, 33].

Current use of low-dose aspirin was more common among gout patients than controls. However, after adjusting for other covariates, including history of ischemic heart disease, low-dose aspirin was not associated with an increased risk of gout. This suggests that the independent pathogenetic role of low-dose aspirin use, if any, may be minor on the risk of incident gout. Our study confirmed an increased risk of gout among users of cyclosporine (an immunosuppressant drug indicated in organ transplant, psoriasis, rheumatoid arthritis, and nephrotic syndrome), whereas there was no association with use of azathioprine or other medications indicated after organ transplant [4, 34, 35]. The apparent association between NSAID use and risk of gout was restricted to patients recently started before the recorded diagnosis of gout. This observation suggests substantial confounding by indication, where use within the first month could be a proxy (treatment) for early manifestations of gout, although a true association cannot be ruled out entirely.

In terms of anti-gout medication use during recent years, we found that allopurinol use had the highest frequency, followed by colchicine, whereas a small proportion of gout patients were taking uricosuric drugs. Treatment patterns did not change in recent years, with only colchicine showing an increase. While allopurinol use seems to be in line with a previous study based on the GPRD between 1990 and 1999, colchicine use appears to have increased since the 1990s, when the authors reported an annual frequency of colchicine use of only 1 to 3% [1]. In addition, it should be noted that the greater proportion of colchicine use could also be explained in part by the fact that our study is based exclusively on individuals newly-diagnosed with a first gout attack, when colchicine prophylaxis is used more often [6].

The strengths and limitations of our study deserve comment. Our study was performed with a large population-based database which contains computerized information entered by primary care physicians that permits the extrapolation of results to the general population. Also, under our design of incidence density sampling, the OR is an unbiased estimator of the incidence rate ratio. Some level of misclassification is unavoidable when working with computerized databases; however, the impact of non-differential misclassification would have most likely biased our estimates of effect toward the null and would not explain the strong associations and dose-response relationships observed in our study. Furthermore, in a secondary analysis when we restricted gout cases to those with GPs' diagnoses of gout combined with anti-gout medication use, our results remained similar (data not shown).

In conclusion, our findings suggest that the contemporary disease burden of gout remains substantial in the UK. Previously identified risk factors still pose considerable relative risk in this cohort, including alcohol intake, obesity, chronic renal impairment, several cardiovascular conditions, and diuretic use. In addition, we found independent associations with prior history of ischemic heart disease, hyperlipidemia, heart failure and psoriasis, and with the use of cyclosporine. These data support the notion that appropriate pharmacologic management and control of lifestyle factors such as maintenance of a healthy BMI, alcohol consumption, and comorbidities, including hypertension and chronic renal failure, would reduce the burden of this common and excruciatingly painful inflammatory arthritis.