What is the probability that a well appearing 18 month old caucasian female infant with fever of 102.2 F (39 C) and URI symptoms has a concurrent UTI? Would you obtain a urinalysis to r/o UTI?

3% - 8%.

One proposed diagnostic algorithm for febrile female infants aged 3 to 24 months suspected of having a UTI.  Click image to enlarge.

Source

Singh, R. and Carpenter, C.  "Does This Child Have a Urinary Tract Infection?"  Annals of Emergency Medicine.  May 2009.

Shaikh N, Morone NE, Lopez L, et al. "Does this child have a urinary tract infection?" JAMA 2008; 298:2895.