The thickness of the fluid layer examined by urinary microscopy is determined by the volume of resuspended sediment put on the glass slide and the size of the coverslip. Using 13-15ul of sediment and coverslips of 18x18mm2 as suggested in European (1) and Japanese (2) guidelines you get a fluid layer thickness of around 40-46um.
Most regular structures of the urinary sediment are thinner than that. Ocasionally though, you can encounter casts -usually of the waxy variety- that have bigger diameters than fluid layer thickness. This can result in them being flattened out under the coverslip with resulting changes in their appearance. Although such flattened waxy casts retain a slightly higher optical density than hyaline casts, they lose their shiny edges with phase contrast microscopy and do not resemble classical waxy casts at all.
When dealing with very crowded sediments, many practitioners put less resuspended fluid on the slide, to thin out the sediment structures. With these sediments it is especially important to be aware of possible flattening artifacts.
Post By Florian Buchkremer
Edited by Anna Gaddy
1. European Urinalysis Group: European urinalysis guidelines. Scand J Clin Lab Invest Suppl. 2000; 231: 1-86
2. Japanese Association of Medical Technologists; Editorial Committee of the Special Issue: Urinary Sediment: Aims of the Guidelines on Urinary Sediment Examination Procedures Proposed by the Japanese Committee for Clinical Laboratory Standards (JCCLS). Japanese Journal of Medical Technology. 2017; 66(J-STAGE-1): 9-17.