Making the diagnosis of a urinary tract infection (UTI) in young infants is not always easy. Infants do not express themselves in the same ways older children and adults do. They usually have non-specific symptoms, including unexplained fever (just like your four-month-old did), vomiting, irritability, and lethargy.
Collecting specimens of urine to study in the lab (urine culture) is at best difficult, since bag-collected specimens in girls do not give very reliable results. It is often necessary to use some invasive method like putting a small tube into the bladder through the urethra or directing a needle into the bladder from the outside and aspirating urine in this way.
Infections of the bladder and sometimes the kidneys are common in children. The prevalence of UTI in infants between two months and two years is as high as five percent and more common in girls. Having a high index of suspicion for a UTI in infants is important since this young age group is at highest risk for kidney damage from such an infection. Once the diagnosis is accurately made, the correct antibiotic can be used to treat the germ causing the infection and your infant can be properly evaluated and followed closely for recurrences.