What is Crithidia Luciliae (and what do we use it for)?

Recently, I saw a patient with suspected SLE who had a lab test by the name of "Crithidia Luciliae" and it was negative. In a limited and completely unscientific survey of a few colleagues, nobody was sure what Crithidia Luciliae was, so the short answer is below.

Crithidia are members of the trypanosome protozoa, just like Trichomonas vaginalis. The kinetoplast (in the tail) of the unicellular flagellate Crithidia luciliae is an excellent source of circular dsDNA and provides a substrate when screening for anti-dsDNA autoantibodies.

The presence of these antibodies is almost exclusively associated with a diagnosis of SLE. The anti-dsDNA antibody sensitivity for diagnosing SLE is 66 to 95 percent, specificity is 75 to 100 percent. The titer correlates with disease activity and is a useful marker, both to assess disease severity and to monitor response to therapy.

There are commercially available testing kits using indirect immunofluorescent assay.

References:
Diagnosis and differential diagnosis of systemic lupus erythematosus in adults. UpToDate 15.3.
Image source: Crithidia Luciliae. Bindingsite.co.uk (permission to display, copy, distribute and download for non-commercial use only).
Wikipedia links.