A brief introduction is given to various inferential methods for the determination of total filtration of x-ray tube assemblies. The HVL method, which is most commonly used, was investigated in detail. The influence of a number of parameters on the relationship between filtration and HVL was systematically studied. The results clearly show that the HVL method only provides a rough estimate for filtration with errors of typically +/- 30%. As a consequence, quality assurance checks on the acceptance and status test level must consider the limited accuracy of this method. Changing requirements from minimum filtration to minimum HVL permitted at particular kVp does not considerably improve the situation.