Background and aim: Reduced renal function is nearly a universal complication experienced by organ transplant recipients, in part related to nephrotoxicity of calcineurin inhibitors. With the promise of new immunosuppressant medications without nephrotoxicity, it is important to accurately understand the extent to which renal function decreases with age. The aim of the study was to model the effect of aging on estimated glomerular filtration rate (eGFR) in the general population.

Methods: The National Health and Nutrition Examination Survey (NHANES) data for 1990 through 2002 incorporated serum concentrations of cystatin-C in addition to serum creatinine. The CKD-EPI creatinine-cystatin C equation was used to calculate eGFR. The multivariable linear regression analysis was performed, taking into account the stratified sampling scheme, to model eGFR with age, hypertension and diabetes mellitus (DM). Hypertension was defined by (1) the average systolic blood pressure of 140 mmHg or higher, or diastolic blood pressure of 90 mmHg or higher or (2) self-reported history of antihypertensive prescription. DM was defined by (1) self-reported history of the diagnosis or (2) current use of insulin or oral hypoglycemic agents.

Results: Out of 21,004 participants of the NHANES surveys, which projected to approximately 230 million in the US population, serum cystatin-C was measured in 5,638. In the univariate analysis, eGFR decreased with age at a rate of 0.77 ml/min/1.73 m² (p<0.01). Hypertension and DM also significantly decreased eGFR (p<0.01). In the multivariable model, DM was no longer significant, whereas a statistically significant interaction was found between age and hypertension – indicating hypertension accelerated age-dependent eGFR loss. The table describes the final model. For example, a 40-year old person with no hypertension is expected to have eGFR of 102.3 ml/min/1.73 m², which is expected decrease by 15.4 ml/min/1.73 m² in a 60-year old if without hypertension and by 21.7 ml/min/1.73 m² if with hypertension.

Conclusion: This model allows prediction of the natural decline in GFR based on age and hypertension and may be useful in assessing the natural evolution of renal function in organ transplant recipients.

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