Image alterations in the teaching microscope. A source of error in synovial fluid crystal identification.

An erroneous identification of sodium urate and calcium pyrophosphate crystals in synovial fluid may occur when utilizing compensated polarized microscopy with a dual-viewing-head teaching microscope. This is due to alteration of the observed image orientation with respect to the actual orientation of the crystal in space. By viewing a simple geometric figure through each microscope head, two basic types of image alterations are identified: a 180-degree rotation and a mirror-image transformation. Combinations of these two may also occur. This problem is clarified and illustrated and suggestions are offered to avoid erroneous crystal identification.