Extending the cystatin C based EKFC-equation to children - validation results from Europe

A new cystatin C based European Kidney Function Consortium (EKFC_CysC) equation was recently developed for adults, using the same mathematical form as the previously published full age spectrum creatinine based EKFC-equation (EKFC_Crea). In the present study the cystatin C based EKFC-equation is extended to children, by defining the appropriate cystatin C rescaling factor Q_CysC.

Rescaling factor Q_CysC for cystatin C was defined as: a) 0.83 mg/L, exactly as it was defined for young adults in the adult equation, and b) a more complex Q_CysC-age relationship based on 4^th degree cystatin C-age polynomials after evaluation of data from Uppsala, Stockholm and Canada and aggregated data from Germany. The EKFC_CysC equation was then validated in an independent dataset in European children (n = 2,293) with measured GFR, creatinine, cystatin C, age, height and sex available.

The EKFC_CysC with the simple Q_CysC-value of 0.83 had a bias of -7.6 [95%CI -8.4;-6.5] mL/min/1.73 m² and a P30-value of 85.8% [95%CI 84.4;87.3] equal to the EKFC_CysC with the more complex 4^th degree Q_CysC-value. The arithmetic mean of the EKFC_Crea and EKFC_CysC with the simple Q_CysC of 0.83 had a bias of -4.0 [95%CI -4.5;-3.1] mL/min/1.73 m² and P30 of 90.4% [95%CI 89.2;91.6] similar to using the more complex 4^th degree Q_CysC-polynomial.

Using exactly the same Q_CysC of 0.83 mg/L, the adult EKFC_CysC can easily be extended to children, with some bias but acceptable P30-values. The arithmetic mean of EKFC_Crea and EKFC_CysC results in bias closer to zero and P30 slightly over 90%.