C/D Ratio in Long-Term Renal Function

Tacrolimus is an immunosuppressive drug. Its C0 concentration, commonly used for monitoring, does not always correspond to its pharmacologic effect. Thölking et al developed an indicator, the C/D ratio, that describes the drug's metabolism rate. Our purpose was to determine whether the points dividing the patients into fast, intermediate, and slow metabolizers that were assumed by those authors would be similar for long-term follow-up after renal transplantation (RTx).

We examined the C/D ratio in 571 patients at their most recent appointments-1 year and more after renal transplantation. The mean time after RTx was 84 months. We studied kidney function both at the most recent appointment and early after RTx.

The median C/D ratio for our group was 1.68. Our observations revealed a negative correlation between the C/D ratio and creatinine concentration and a positive correlation between the C/D ratio and eGFR concentration long term after RTx. We formulated a C/D ratio cutoff point between an eGFR < and ≥ 60 mL/min/1.73 m² and came up with the value of 1.53. It was found that between the < 1.53 and ≥ 1.53 groups, there were significant differences in creatinine and eGFR concentrations at the most recent appointment, as well as differences in how creatinine and eGFR levels varied over time between RTx and the most recent observation.

The C/D ratio is useful for assessing the effect of the tacrolimus metabolism rate on long-term renal function. We propose the C/D ratio value of 1.53 as the cutoff point below which the ratio provides a negative prognosis for long-term renal function.