Giardiasis in North West England: faecal specimen requesting rates by GP practice

Abstract

Many cases of giardiasis in the UK are undiagnosed and among other things, diagnosis is dependent upon the readiness of GPs to request a specimen. The aim of this study is to assess the rate of specimens requested per GP practice in Central Lancashire, to examine the differences between GP practices and to estimate the pattern of unexplained spatial variation in the practice rate of specimens after adjustment for deprivation. To achieve this, we fitted a set of binomial and Poisson regression models, with random effects for GP practice. Our analysis suggests that there were differences in the rate of specimens by GP practices (P < 0·001) for a single year, but no difference in the proportion of positive tests per specimen submitted or in the rate of positive specimens per practice population. There was a difference in the cumulative rate of positive specimens per practice population over a 9-year period (P < 0·001). Neither the specimen rate per practice for a single year nor the cumulative rate of positive specimens over multiple years demonstrated significant spatial correlation. Hence, spatial variation in the incidence of giardiasis is unlikely to be confounded by variation in GP rate of specimens.

Keywords: water-borne infections.

Fig. 1.

Index of multiple deprivation (IMD) scores by GP practice. Larger circles correspond to higher IMD scores, shading of dark grey to white circles indicates low to high IMD scores.

Fig. 2.

Histograms of main output variables showing the spread of values for different GP practices.

Fig. 3.

Rate of specimens in 2011 per 1000 population by GP practice. Larger circles correspond to higher rates of specimens, shading of dark grey to white circles indicates low to high rates of specimens.

Fig. 4.

Rate of positives in 2003–2011 per 1000 population by GP practice. Larger circles correspond to higher rates of positives, shading of dark grey to white circles indicates low to high rates of positives.

Fig. 5.

Rate of positives in 2011 per 1000 population by GP practice. Larger circles correspond to higher rates of positives, shading of dark grey to white circles indicates low to high rates of positives.

Fig. 6.

The rate ratio of positive specimens per 1000 practice population in each year and positive specimens per 1000 practice population in 2003, with 95% confidence intervals. A rate ratio of 1 indicates no difference between the rate of positive specimens in that year and the rate of positive specimens in 2003.

Fig. 7.

The rate ratio of positive specimens in 2003–2011 per 1000 practice population in each week (1–52) and positive specimens in 2003–2011 per 1000 practice population in week 0, showing seasonality. A rate ratio of 1 indicates no difference between the rate of positive specimens in 2003–2011 per 1000 practice population in that week and the rate of positive specimens in 2003–2011 per 1000 practice population in week 0.

Fig. 8.

Variograms of fitted random effects for (a) rate of specimens and (b) rate of positive specimens in 2003–2011, with reference bands. Each point is the average over the pairs of observations in the bin (a small range of distances).