Patients
The sample comprised a group of patients in the Jamaican Sickle Cell Cohort attending the 2004 Annual Sickle Cell Cohort Review conducted at the Sickle Cell Unit Clinic at the University of the West Indies, Kingston, Jamaica. The Jamaican Sickle Cell Cohort includes patients with sickle cell diseases detected using standard criteria during screening of 100,000 consecutive non-operative deliveries at a large maternity hospital (the Victoria Jubilee Hospital) between 1973 and 1981. This screening yielded 580 patients with different sickle genotypes, of which 315 had HbSS disease, 201 Sickle cell-haemoglobin C, 33 Sickle cell-β+ thalassaemia, 14 Sickle cell-β0 thalassaemia and 17 sickle variants. They have been followed clinically since birth [14]. At the 2004 cohort review 267 of 302 patients eligible to attend the annual cohort review were seen. The reasons for being absent were: Twenty-two of the patients did not attend the review that year, 11 were lost to follow up, or incarcerated, and 2 were ill at the time of review. One patient refused to leave a sterile urine sample during the cohort review period, thus the final participating patient sample for this study was 266 patients. Ethical approval was granted for this study by the University Hospital of the West Indies, University of the West Indies Faculty of Medical Sciences Ethics Committee.
Measurements
Aseptically collected midstream urines (MSUs) were obtained from the symptom free attendees of 2004 cohort review and sent on the same day for microbiological culture. Urinalysis was performed on aliquots of the MSU specimen using the QuickVue UrinChek™10+SG test strips. Bacterial culture was performed by streaking 0.002 mL of midstream collected urine with a calibrated loop on MacConkey and 5% sheep blood agar plates. These agar plates were incubated at 35°C for 24 hours under aerobic conditions. Isolates were considered significant if there were ≥ 105 colony forming unit/mL (CFU/mL) with 2 or less isolates, doubtful significance if 104 – 105 CFU/mL, insignificant if < 104 CFU/mL. Mixed growths, in any count, of more than two organisms were considered to be contaminated. Significant isolates were selected for identification and antimicrobial susceptibility testing using the vitek (biome'rieux' version R06.01, Missouri USA). Proteinuria was defined as a protein reading of trace or greater on dipstick urinalysis.
Patients complaining of any symptoms, whether related or unrelated to the urinary tract, were not included in the study on that day. Rather, these patients were treated for their complaint and studied on another day when they were without symptoms. Subjects whose urine microbiological report was abnormal were asked to return to the unit for a repeat midstream urine collection and culture.
Blood samples were also taken for the measurement of serum creatinine. Serum creatinine were measured in the Tropical Medicine Research Institute laboratory with an Abbott ALCYON 300I, using the alkaline picrate method.
The clinical records of all cohort patients in the sickle cell unit database were examined and the following information extracted: age, gender, steady state haematology, steady state uric acid levels, history of urinary tract infection, and history of gram negative sepsis.
Definition of asymptomatic bacteriuria
Probable ASB was defined as the presence of at least 105 colony forming units of a urinary tract pathogen per milliliter of urine in a culture of a midstream urine specimen obtained from a patient during the cohort review visit. A confirmed episode of asymptomatic bacteriuria was defined as two or more consecutive cultures with evidence of asymptomatic bacteriuria due to the same urinary tract pathogen with the same sensitivity pattern. A sample was deemed grossly contaminated if it grew more than two urinary tract pathogens even if in significant quantities.
Statistical methods
Values are expressed as counts or means ± sd as appropriate. The prevalence of probable ASB and confirmed ASB were determined as the ratio of the number of urines classified as probable ASB and confirmed ASB, to the total number of collected MSU samples. The 95 percent confidence interval (95%CI) for this proportion was computed according to the Wilson score method without continuity correction [15]. For continuous outcome variables differences in means between ASB status (confirmed ASB group vs. sterile urine group) and genotype were determined by two factor analysis of variance. Associations between categorical exposures and ASB status were adjusted for genotype by Mantel-Haenszel methods. Logistic regression was used to examine the relationship between potential predictors (age, history of urinary tract infection, genotype, steady state haematology, uric acid and serum creatinine) and probability of having ASB. Likelihood ratio tests were used to determine which predictors would be included in the final model. We assessed differences in the frequency of past attendance for symptomatic urinary tract infection between ASB status and genotype using a negative binomial regression model. Clinic visits less than 14 days apart were considered the same event. Visits were assumed to be independent between individuals but repeat visits by individuals were not assumed to be independent. Data were analyzed using the Stata statistical software version 8 (Statacorp, TX, USA). Tests were considered significant if p < 0.05.