Topic Discussion: Pyelonephritis but no Urinary Tract Infection?

Pyelonephritis is defined as neutrophilic infiltration within the interstitium suggestive of a bacterial cause of urinary tract infection that might have migrated to the kidney. It rarely evolves into an abscess.

Can this exist without any signs of an urinary track infection? 
A study published in NDT looked prospectively of over 200 cases of acute pyelonephritis. 
What did they find?

Urinary culture was only positive in 31% of patients and blood cultures in 21%

92% did have CT findings of pyelonephritis. 
No differences were noted in patients with positive or negative CT findings in terms of fevers, and wbc counts, pyuria, urine cultures and symptoms. 

Why the negative urinalysis and urine culture? 

The low frequency of positive urine culture may be explained by previous antibiotic treatment, either self-prescribed or prescribed by the general practitioner, and by the possibility that infection was confined to the renal parenchyma. Could reflux disease explain some findings?While the association between acute pyelonephritis  and reflux has been extensively studied in children, the literature does not indicate when reflux must be searched in adults. The authors performed retrograde urethrocystography in the case of recurrent acute pyelonephritis or in the presence of urinary cavities dilation or urinary tract abnormalities: they found reflux in 20.9% of patients. 

In other words, the absence of infected urine does not rule out the diagnosis of acute pyelonephritis in common clinical practice. Renal abscesses are frequent and need to be looked for. Hence, it seems advisable to systematically perform CT or MRI, which have greater sensitivity than ultrasound in detecting them.