Identifying Crystals in the Urinary Sediment
Crystalluria is a common finding in the urinary sediment. Although its clinical significance is oftentimes limited, most people have a natural desire to identify these frequently nifty structures. Unfortunately this is a rather difficult affair. Osler's most famous dictum about medicine in general ("...science of uncertainty...art of probability") perfectly sums up microscopic crystal identification in the urine as well.
Fig. 1. Different forms of uric acid crystals (all by bright field microscopy).
The basic problem relates to the nonspecific nature of many crystal morphologies: As a general rule the same substance can form many different crystals (Fig. 1), and very similar crystals can consist of completely different compounds (Fig. 2).
Fig. 2. Dumbbell-shaped crystals. Left composed of uric acid, right calcium oxalate monohydrate. (upper part bright field, lower left phase contrast, lower right polarized light).
Without any means of physicochemical verification, the accurate assignment of crystals really is a challenge. It depends on a thorough integration of diverse properties:
- Morphology (form, structure, color, size) by phase contrast and bright field microscopy
- Birefringence (intensity, color) in polarized light +/- red compensator
- Surroundings (accompanying crystals, bacteriuria/leucocyturia, amorphous material...)
- Urinary pH
With careful consideration off all these factors, many crystals -especially when abundantly present- can be identified with clinical certainty. In contrast more sporadic and/or very nonspecific crystalline structures frequently have to go unidentified. The ease with which crystals are sometimes ascribed to certain substances or drugs does not measure up to the scope of the problem and amazes me time and again.
I highly recommend allowing oneself enough room for error and uncertainty, when trying to identify crystals in the urinary sediment.