The determination of the role that drugs may have played in a death is an important part of the investigation into unexplained deaths. Renal impairment may lead to a reduction in drug excretion rate and therefore an accumulation of drugs or metabolites, leading to possible toxic or lethal effects. Creatinine levels are known to be stable in the post mortem period and in life can give an indication of kidney function. There are however widely reported limitations when using creatinine in isolation and so we investigated the usefulness of using estimated glomerular filtration rate (eGFR) for scoring an individual as having renal impairment using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formula. We analysed unpreserved vitreous for creatinine in 812 individuals using an isotope dilution mass spectrometry (ID-MS) traceable enzymatic. We found that the biochemical analysis of post mortem vitreous creatinine and subsequent calculation of eGFR is a useful adjunct to the standard testing that takes place during a post-mortem examination and can assist in death investigation. Using an eGFR of <60 mL/min/1.73 m2 gave a sensitivity of 94.3% and specificity of 97.3% when scoring an individual as having renal impairment. We therefore recommend the calculation of eGFR for the determination of possible renal impairment in post mortem investigations. It is, of course, always pertinent to interpret any results using a wealth of case information. Extreme caution should be exercised in cases where insufficient clinical information/history is available, particularly in cases in which there is suspected diabetic ketoacidosis, dehydration or hospitalisation prior to death.