Inulin is used in analysis of glomerular filtration rate. It is not naturally found in the body, is non-toxic, and is usually infused rather than, say, taken orally.

The reason it is used in this sort of analysis is that it is freely filtered by the kidneys, and is neither reabsorbed, nor secreted.

The equation used to find the GFR is

GFR = (U * V) / P
Where GFR is equal to the urine inulin concentration times the volume flow of urine, divided by the plasma inulin concentration.

Finally, make sure you don't confuse inulin with insulin.

These are my interpretation of my lecture notes, but I may have used some references from Hole's Anatomy and Physiology (Shier, Butler, Lewis) and Human Physiology (Vander, Sherman, Luciano)