How to assess PH assumption in extended Cox model?

kivanvan — Tue, 01 Mar 2022 16:07:09 GMT

In my PH Cox model (using PROC PHREG), one of my continuous covariate (EGFR) violates the PH assumption. Based on the plot of Schoenfeld residual below, I decide to use a heaviside function with the time cut-off at 2 (the 600th rank of time) on this variable.

I believe that, with the heaviside function, I still assume the proportional hazard in each time interval. I tried the ZPH option and ASSESS statement, and I got the note: "Model assessment is not available when there are time-dependent covariates. The ASSESS statement is ignored." I further tried to output Schoenfeld residual (see code below), but I got no observation in the output dataset. Does anyone know how to assess the PH assumption in this model?

 PROC PHREG DATA= DF;
  CLASS EXPO SEX;
  MODEL TIME*SA(0,)= EXPO AGE SEX  EGFRT1 EGFRT2 / RL=PL;
   EGFRT1=0;
   IF TIME<2 THEN EGFRT1=EGFR;
   EGFRT2=0;
   IF TIME>=2 THEN EGFRT2=EGFR;
  OUTPUT OUT=SCHOEN RESSCH=SCHEXPO SCHAGE SCHSEX SCHEGFRT1 SCHEGFRT2;
RUN;

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How to assess PH assumption in extended Cox model?