Urine Sediment of the Month: Cystine Crystals

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Cystinuria is the most common kidney stone disease with Mendelian genetics. Caused by mutations in SLC7A9 and SLC3A1, affected patients excrete high amounts of cystine in their urine and are vexed by recurrent episodes of nephrolithiasis.

Cystine stones (Fig. 1) are aggregates of individual crystals with hexagonal habits. In the urinary sediment pathognomonic hexagonal plates with large basal surfaces can be found (Fig. 2). 

Fig. 1. Cystine stones. (Scale 0.5 cm)
Fig. 2. Typical hexagonal cystine crystals. (Phase contrast, bright field, original magnification x400)

Although cystine crystals in the urine may form complex aggregates (Fig. 3 and 4), the underlying hexagonal structure is usually readily discernible. Characteristically, they are colorless with bright field microscopy and the crystal margins are perfectly linear (Fig. 5). Phase contrast might create some color artifacts (Fig. 6 and 7).

Fig. 3. Huge cystine compound. (Phase contrast, original magnification x400)
Fig. 4. Crystal aggregates in low power field. (Bright field, original magnification x100)
Fig. 5. Multi-layered crystal. (Bright field, dark field, original magnification x400)
Fig. 6. Color artifacts 1. (Phase contrast, original magnification x400)
Fig. 7. Color artifacts 2. (Phase contrast, original magnification x400)

Depending on the thickness of the crystals, their birefringence tends to be of low to moderate intensity (Fig. 8). Very thin crystals are easily missed, especially by bright field microscopy alone (Fig. 9).

Fig. 8. Crystals under polarized light. (Original magnification x400)
Fig. 9. Thin and small crystals can be easily missed. (Phase contrast, bright field, original magnification x400)

Cystine crystals in the urinary sediment (a) confirm the presence of cystinuria and (b) demonstrate urinary supersaturation of cystine.

While perfectly specific, the sensitivity of cystine crystalluria for the presence of cystinuria is quite low. 

Post by: Florian Buchkremer

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