Case #218 - December, 2007

The parasitemia for this case of babesiosis was approximately 23%. For this case, a stained thin smear was examined and parasitemia was expressed as the percentage of red blood cells (RBCs) infected, not the number of parasites observed. Counting the number of parasites could lead to an inaccurately high value and may lead to more aggressive therapy than necessary. When using this approach, report the number of infected RBCs per 100 RBCs; a minimum of 500-2,000 RBCs should be counted. Due to the constraints of presenting this as a case study, we only showed three images with about 30 RBCs each. The values for the images should be approximately as follows:

• Figure A – 6 infected RBCs out 28 RBCs
• Figure B – 9 infected RBCs out of 29 RBCs
• Figure C – 5 infected RBCs out of 29 RBCs

These counts give an average of 6.6 infected RBCs per 28.6 RBCs or 23% of RBCs infected. In areas endemic for malaria, the thick blood smear is generally used for calculating parasitemia: if the white blood cell count is known, the parasitemia can be estimated as the number of parasites per microliter of blood. Another approach that can used for either thick or thin smears is to count the number of parasites per 100 white blood cells (WBCs); this number can also be converted to the number of parasites per microliter of blood.

More on: Calculating Parasitemias