The robustness of a dual-beam, optical null, Fouriertransform near-infrared (FTNIR) spectrometer was investigated by, means of online, near-infrared measurements and predictions of urea concentrations in spent dialysate during hemodialysis treatment. Simple multivariate calibration using a few factors based on a small number of prepared samples provided stable and accurate predictions over a period of 1 month. The calibration was robust when faced with adjustment of reference cell intensity and did not require a daily measured reference spectrum. The root-mean-square error of prediction of urea was 0.4 mM based on a two-factor partial least-squares regression model.