Clinical laboratories may systematically apply factors to assay results after analysis, but before reporting, in order to facilitate comparison of data from different methods. This may be done to align with other patient results, reference intervals or clinical decision points. These factors, which we term Post Analytical Correction Factors (PACF), may be applied to all types of results derived from the method, i.e. quality control (QC) and external quality assurance (EQA), as well as the patient results. As the principal use of PACF is comparing patient results, it is important that the laboratory use commutable materials (i.e. patient samples) and a formal process to establish, apply and manage PACF. We report on preliminary guidelines for PACF from a recent workshop.