Sunday, August 10, 2014

Answer to Case 314

Answer:  Not a parasite:  uric acid crystals

Crystals are commonly seen in urine specimens and can be a very good mimic for parasite eggs.  This is particularly true for the 'lemon-shaped' uric acid crystals which mimic Schistosoma haematobium eggs.  You can differentiate uric acid crystals from S. haematobium eggs by the following features:

1. Uric acid crystals vary in size and shape and are often much smaller than S. haematobium eggs. In contrast, S. haematobium eggs are regular in size and shape, and quite large (approximately 150 micrometers in length).
2. Uric acid crystals commonly have points on both ends instead of the single 'pinched-off' spine of S. haematobium eggs. They can also have lateral points or take on other shapes.
3. There are no internal parasite structures in crystals
4. Finally, crystals often fracture and break, although this is not shown in this case.

Here is a picture from this case that demonstrates many of these features, including points at both ends, variability in size and shape and lack of internal parasite structures.

Thank you all for writing in on this case!  Crystals can be challenging, so I like to show them whenever they come up in my lab.

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