We developed a method for the analysis of creatinine in dried blood spot (DBS) samples to facilitate monitoring of renal function in combination with TDM of immunosuppressive drugs for transplant patients outside the hospital. An 8-mm disc of the DBS was punched, extracted and followed by LC-MS/MS analysis. The haematocrit proved to have a significant influence on the analysis of creatinine in DBS samples. As potassium is a suitable marker for haematocrit, we implemented a method for measuring potassium in DBS and correct the creatinine for haematocrit. For both creatinine and K(+) in DBS analytical and DBS, validation was performed, both components met the validation criteria and no other influences beside the haematocrit were detected. To assess the haematocrit correction, samples were compared to the 'golden' standard and plotted before and after correction. The correction showed a great improvement in agreement between the DBS assay and venous blood assay.