Answer to Case 728

 Answer to the Parasite Case of the Week 728: Not a parasite egg; uric acid crystals.

Uric acid crystals are a very convincing mimic of Schistosoma haematobium eggs! They are both found in urine, and they both have a terminal spine. However, there are a number of features that can be used to easily differentiate the two:

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 (i.e., miracidium) in uric acid crystals
4. Finally, crystals often fracture and break, and may have irregular contours.

Here is a nice side-by-side comparison of true Schistosoma eggs (left) and uric acid crystals (right):

I've featured uric acid crystals several times before on this blog, so I thought I would take this opportunity to highlight images from past cases. As you can see from the images below, there is a variety of appearances that uric acid crystals can take in urine:

Case of the Week 464

Case of the Week 41

Case of the Week 160

Case of the Week 295

Case of the Week 314