Urine Sediment of the Month: Common Crystals!

Crystals are a frequent finding in the urinary sediment. Besides microcrystalline deposits (amorphous phosphates and urates), the most common types of crystals in my practice are:

1. CaOxalate dihydrate (COD)
2. CaOxalate monohydrate (COM)
3. Uric acid
4. Struvite

Figure 1. CaOxalate dihydrate crystal with the typical octahedral or bipyramidal structure. A Pseudo differential interference contrast image. B Oblique view of a crystal model. C Under the microscope COD crystals are usually looked at from above (or below), because the “height“ of the pyramids is usually rather low. This gives rise to the typical envelope appearance. D View from the side.

Figure 2. CaOxalate dihydrate crystals amid squamous epithelial cells. (Bright field, original magnification x100)

Figure 3. Huge calcium oxalate dihydrate crystal. (Bright field, original magnification x400)

In their more typical forms they can be readily identified by microscopic morphology, their appearance under polarized light, and urinary pH.

CaOxalate DiHydrate crystals: COD crystals usually display a bipyramidal structure (Fig. 1). With bright field microscopy these resemble envelopes (Fig. 1C, 2, 3), which do not exhibit any significant birefringence. They are mostly independent of urinary pH, but are less common in alkaline urine. Their clinical relevance is limited, as they are often encountered in perfectly healthy people. In contrast to COM crystals they develop in conditions with higher ratios of calcium to oxalate.

CaOxalate MonoHydrate crystals: COM crystals show a much more varied morphology. Dumbbell shapes (Fig. 4, 5) are known best, but ovoids, often with a navel, are much more common (Fig. 6). COM crystals show intense birefringence (Fig. 5C, 6B). As for COD their development is not significantly influenced by urinary pH values. Exclusive COM crystalluria should alert you to the presence of hyperoxaluric conditions.

Figure 4. Abundant CaOxalate monohydrate crystalluria, mostly of the dumbbell-shaped type. (Bright field, original magnification x100)

Figure 5. CaOxalate monohydrate crystal. A Pseudo differential interference contrast. B Bright field. C Polarized light. D Phase contrast. (Original magnification x400)

Figure 6. Ovoid CaOxalate monohydrate crystals. A Bright field. B Polarized light. (Original magnification x400)

Figure 7. Uric acid crystalluria in a patient with volume depletion. (Bright field, original magnification x100)

Uric acid crystals: An even wider spectrum of morphologies can be found with uric acid crystals (Fig. 7 and 8). Typical shapes include barrels, rhomboids, rosettes. They are frequently amber colored (Fig. 8) and show intense and often polychromatic birefringence (Fig. 9). Uric acid crystals are encountered in acidic urine only.

Struvite crystals: They lie at the other end of the pH spectrum and are found in alkaline urine, often due to infections with urease splitting organism. Typical crystals show a characteristic coffin-lid morphology (Fig. 10 and 11) with only weak to moderate birefringence. Somewhat tilted forms are also common (Fig. 12).

Figure 8. Huge crystals of uric acid. (Bright field, original magnification x400)

Figure 9. Uric acid crystals with magnificent polychromatic birefringence under polarized light. (Courtesy of @JoseTesser)

Figure 10. Elegant struvite crystal. (Bright field, original magnification x400)

Figure 11. Struvite crystal with the typical coffin-lid appearance in a patient with distal RTA and a urinary tract infection. (Bright field, original magnification x400)

Figure 12. Tilted struvite crystal. A Phase contrast. B Bright field. (Original magnification x400)

Given the wide morphological spectrum of each crystalline category, accurate classification of unusual crystals in the urinary sediment can be difficult, if possible at all. Confirmation may require more sophisticated methods like infrared spectroscopy (FTIRM).