Answer to Case 295

Answer:  Uric acid crystals

This was a challenging case given that uric acid crystals are a very convincing mimic of Schistosoma hematobium eggs.  Fortunately they can be differentiated by a number of features:

1. The crystals are pointed at both ends while S. hematobium eggs have a single (terminal) spine.
2. The pointed ends of the crystals are more triangular, while true S. hematobium spines have a pinched-off appearance.
3. Crystals are smaller than Schistosoma hematobium eggs (usually < 100 micrometers in length; compare their size to the surrounding epithelial cells) and vary significantly in size.  In contrast, S. hematobium eggs are large (112 - 170 x 40-70 micrometers).
4. There are no internal staining structures on Papanicolaou stain, unlike what would be seen with true eggs (see example below).
5. Crystals are often cracked, whereas true eggs rarely crack but instead may be crushed or deformed
6. Crystals are very birefringent on polarized light unlike true eggs which do not polarize to this extent.

These features are demonstrated by the following images:

Comparison of true S. hematobium egg (left) and crystal (right) on Papanicolaou stain:

Uric acid crystals may be seen in urine in a number of settings, particularly in acidic urine and certain metabolic states such as gout.  

From Blaine Mathison:

These ellipsoid-shaped structures might look like schisto

but really they're uric acid crystals, dont ya know

high concentrations in blood might lead to gout

so from your diet leave excess sugars out

or excruciating pain you may feel down in your toe!