Study Design
This is a cross-sectional retrospective review of 73,001 presentations to Blacktown or Mt Druitt Emergency Departments, from mid-2016 to 2018, inclusive. These patients were administered a blood test as part of their presentation and the HbA1c (glycated haemoglobin A1c) was recorded into the database, which shall be referred to as the HbA1c database. The database also recorded various demographic data and the estimated Glomerular Filtration Rate (eGFR) for each presentation and other variables. The database is the product of a testing regime set up as routine care for patients attending the two ED’s and has been running since 2016. The full testing protocol has been published elsewhere(11).
Outcome of Interest
The outcome of interest is patients with potential kidney dysfunction and diabetes. Individuals with diabetes were identified based on HbA1c values and kidney dysfunction was defined as Stage 2 or higher based on eGFR. Those that meet these criteria are referred to as individuals with DM associated CKD for the purposes of this paper. The outcome variable is a coded binary variable for analysis.
Inclusion/Exclusion Criteria
Patients were included in the study if they had an HbA1c result and an eGFR result, along with the ethnicity recorded in the HbA1c database.
Patients’ repeat presentations to the emergency department were also recorded into the database but these were excluded from the study so as to keep only unique presentations.
Any patients who didn’t have their ethnicity recorded, i.e. not admitted into the hospital, were also excluded from the study database.
Study Population
The following diagram shows the inclusion and exclusion of patients in the study.
Co-variates
The HbA1c database recorded a number of different variables for each patient. For the purposes of this study, only the follow variables were considered.
- HbA1c
- eGFR
- Gender
- Age
- Ethnicity
- Marital Status
- Smoking Status
- Aboriginal/Torres Strait Islander Status
- SEIFA score (Socio-Economic Index for Areas)
- Postcode
- Body Mass Index (BMI)
Analysis
Data was analysed by importing relevant variables and datasets into STATA 16 statistical analysis software package. A logistical regression model was fitted to variables included for the analysis and the significance level was set at p <0.05. Due to the size and nature of the database, missing values were ignored in the analysis.
Variables
The eGFR result will be used to differentiate patients into stages of kidney disease as per the World Health Organisation (WHO) staging of renal impairment as per Table 1. Glycaemic categorise were allocated as per Table 2.
Table 1: eGFR and Stages of Renal Impairment
eGFR Result
|
Stage of Renal Impairment
|
> 90 mL/min
|
Stage 1: Normal
|
60-89 mL/min
|
Stage 2: Mild
|
45-59 mL/min
|
Stage 3A: Moderate
|
30-44 mL/min
|
Stage 3B: Moderate
|
15-29 mL/min
|
Stage 4: Severe
|
< 15 mL/min
|
Stage 5: End
|
Table 2: HbA1c and glycaemic status
HbA1c(%)
|
HbA1c mmol/mol
|
Glycaemic Status
|
< 5.7%
|
< 39 mmol/mol
|
Normal
|
5.7-6.4%
|
39-47 mmol/mol
|
Prediabetes
|
> 6.5%
|
> 47 mmol/mol
|
Diabetes
|
The main explanatory variable for the study will be ethnicity, using the patient’s country of birth as a surrogate measure. Other variables will be included in the analysis based on a review of past literature and a univariate analysis indicating significant correlation with diabetes associated CKD.
Socio-Economic Status was determined by importing Australian Bureau of Statistics (ABS) SEIFA values for postcodes and the SEIFA values were attached to patient postcodes in the database. SEIFA is an index of four different measures which represent different aspects of economic advantage or disadvantage. One of the SEIFA measures is the Index of Relative Socio-economic Advantage and Disadvantage (IRSAD) which was used for this study measures both advantages and disadvantages within a region.
SEIFA is measured in single unit increments, with a bell curve distribution centred over 1000 as a median value of IRSAD for a given region. For ease of interpretation, SEIFA was converted from single unit increments to 100 unit increments for analysis. A higher IRSAD score indicates a greater relative advantage verses disadvantage for an area. A lower score indicates greater disadvantage verses advantage.
The BMI variable contained many missing variables, with only approximately 12, 000 BMIs’ recorded in the database and as such was not included in the subsequent analysis. A total of 21 missing Hb1Ac observations were also found in the database, which were not included in the analysis.
Smoking status was determined by using the ICD-10 codes F17 and Z72 which cover Nicotine Dependence and Tobacco use respectively. No differentiation was made between current and past smokers for this analysis.
Model Building Process
Variables were selected based on past literature suggesting correlation between DM and CKD. Past studies have found significant correlation between diabetes, ethnicity, age, SES status, marital status and gender, smoking status and Aboriginal/Torres Strait Islander (A/TSI) (7, 12-16). These variables were individually tested for correlation with the outcome variable with significance set at p<0.05. A forward method model building process was used for the analysis.
Patients were grouped together by ethnic and geographic majority as described in Table 3. Certain ethnicities such as South America for example are an amalgamation of patients whose country of birth is in the South Americas. This was done to balance the groups as certain countries were represented by very few patients. Likewise, a Caucasian ethnicity is represented by the Australian/North American/European group in Table 3 and is used as the base ethnicity for analysis. Table 4 contains a breakdown of each CKD stage by absolute numbers.
Ethics
This research project was approved by the Ethics Board (Application Number: 2019/ETH01985) of the Western Sydney Local Health District for the use of anonymised patient data for research and publication.