Abstract
World-wide the incidence of individuals developing stage 3 to stage 5 chronic kidney disease (CKD) is increasing. Diabetic nephropathy is the major aetiological factor for this increasing epidemic of CKD. A significant complication of CKD is the development of a normochromic normocytic anaemia related to a lack of erythropoietin as well as possible erythropoietin resistance along with a reported decrease in red blood cell (RBC) survival. The altered RBC survival may affect the validity of glycated haemoglobin (HbA1c) which is used as the standard assessment of glycaemic control in long term diabetes management.
The uraemic environment of stage 4 and 5 CKD and potentially the dialysis procedure itself may contribute to altered RBC survival. At present, there is inconsistent data and little agreement to what extent the RBC survival is reduced in the setting of CKD, particularly now with the availability of erythropoietin stimulating agents (ESAs) and modern dialysis techniques. Additionally, it is unclear what impact ESAs, now used in the majority of dialysis patients, have on the accuracy of HbA1c. Normal RBC survival is considered to be 120 days, however this value was based on very few data using methods that were inadequate and is for this reason debatable. The available methods to study RBC survival are all limited in their applicability, and no ideal technique exists. Labelling with radioactive chromium (51Cr), currently deemed the gold standard for measuring RBC survival, is limited by unavoidable losses of label from RBCs. When using the 51Cr method, it is necessary to incorporate factors to account for these inevitable losses. However, these losses have not been systematically investigated.
The first part of this thesis was to examine the accuracy of current methods of assessing RBC survival both in vitro and in vivo. Prior to conducting in vivo RBC survival studies, the different pathways of 51Cr loss were explored. To identify and evaluate the unavoidable losses occurring from intact RBCs, blood of healthy and CKD subjects was labelled with 51Cr and disappearance of label was studied under in vitro conditions. Elution and vesiculation were identified as mechanisms of label loss, and are independent of RBC destruction. The major pathway of loss was elution and vesiculation accounted only for minor amounts, further quantification was restricted by the in vitro design of the study.
To determine whether RBC survival is reduced in dialysis patients and differs among dialysis modalities, survival was determined by means of the 51Cr labelling. Relative RBC survivals were measured in haemodialysis (HD) and peritoneal dialysis (PD) patients and compared to healthy subjects. The RBC survival was demonstrated to be significant shortened in dialysis patients when compared to healthy controls, though the observed relative reduction was less than previously stated, it remains to contribute to the anaemia of CKD. Similar RBC survivals were noted in HD and PD patients.
The second part of the thesis examined the impact of decreased RBC survival on the accuracy of glycated haemoglobin (HbA1c) as a marker of glycaemic control in diabetic patients with stage 4 and 5 CKD compared to other assessments of glycaemic control including glycated albumin, fructosamine and continuous glucose monitoring.
In CKD, the alterations in RBC turnover, due to reduced cell survival and/or ESA administration, have an impact on the HbA1c concentration and thereby likely reduce its accuracy. The exact relationship between different markers of long-term glycaemic control and real-time glucose needed to be established. The reliability of HbA1c was investigated in a population with diabetes and severe CKD (CKD stages 4 and 5: glomerular filtration rate < 30 mL/min) by means of continuous glucose monitoring and compared to other markers of glycaemic control, namely glycated albumin (GA) and fructosamine. Average glucose concentrations were underestimated by HbA1c concentrations in diabetic subjects with CKD stages 4 and 5. It was demonstrated that GA was more accurate in reflecting average glucose in predialysis and dialysis patients and should be the preferred marker in the assessment of long-term glycaemic control in this population.
The final aspect of the thesis arose from an interest in the role of insulin as an anti-inflammatory agent and what impact a continuous insulin infusion during a haemodialysis session might have on markers of inflammation which are known to be elevated in haemodialysis patients.
The risk of cardiovascular morbidity and mortality is substantially increased in the dialysis population. Haemodialysis is associated with a pro-inflammatory state and increased oxidative stress which contributes to the increased risk of cardiovascular disease in CKD. Insulin has been demonstrated to exhibit anti-inflammatory actions. A pilot study was undertaken to explore if an insulin infusion during haemodialysis modifies markers of inflammation and oxidative stress. Anti-inflammatory effects of insulin were demonstrated by a reduction of CRP in the immediate 24 hours following a standard dialysis accompanied by an insulin infusion. Further exploration of the relevance of this finding is required.