Acute pyelonephritis and uncomplicated UTI may be differentiated by all of the following, except:

Although acute pyelonephritis may be associated with organisms in excess of 100,000CFU/ml, this test cannot be used to localize the infection to the kidneys as such high counts are also seen in uncomplicated UTIs. All other tests mentioned are used to localize the infections to the kidney. Ref: Harrison's Principles of Internal Medicine, 16th Edition, Page 1717; Diagnosis of Genitourinary Disease By Resnick and Older, 2nd Edition, Pages 275-77