Approach

A report of fever and chills, flank pain, and irritative voiding symptoms (e.g., urgency, frequency, and dysuria) should prompt a workup. Other key symptoms include nausea, vomiting, and dehydration. It is also critical to be aware of signs of sepsis (e.g., tachycardia, tachypnea, hypotension, fever or hypothermia, poor capillary refill, mottled or ashen skin, cyanosis, newly altered mental state, or reduced urine output).[32]

The triad of flank pain, fever, and nausea and vomiting occurs much more often in patients with pyelonephritis than in those with cystitis.[33]

Physical exam

Temperature greater than 100.4ºF (38.0ºC) is a key finding supporting the diagnosis. In one study, temperature greater than or equal to 100ºF (37.8ºC) was strongly correlated with acute pyelonephritis.[34] Tachycardia and hypotension may be present. Costovertebral angle tenderness may be pronounced.

Laboratory tests

Initial laboratory tests in all patients with suspected pyelonephritis are urinalysis and urine culture.[5]​​[35]​​​ Urinalysis shows pyuria, bacteriuria, and varying degrees of hematuria. Do not define microhematuria by positive dipstick testing alone. Microhematuria is defined as three or more red blood cells per high-powered field on microscopy of a properly collected urinary specimen.[36][37]​​ Pyuria is almost invariably present; in fact, its absence should prompt consideration of an alternative diagnosis. WBC casts, if present, suggest a renal origin for the pyuria. A Gram stain performed on spun urine can sometimes help distinguish gram-negative from gram-positive organisms, thus influencing the choice of therapy. The specificity of these tests in patients with acute pyelonephritis is less than 28% with a misdiagnosis rate (with regard to specific site of infection) of 55% of cases.[38]

Urine culture (from a clean-catch or catheterized specimen) shows heavy growth of the causative pathogen (classically ≥100,000 colony-forming units [CFUs] per milliliter of voided urine).[39]

Blood cultures are indicated in more severely ill patients. Blood cultures are positive for the causative pathogen in approximately 10% to 20% of women with acute uncomplicated pyelonephritis.

Other initial laboratory tests indicated in the initial workup are complete blood count, erythrocyte sedimentation rate, and serum C-reactive protein.[40] Procalcitonin (a propeptide produced by the monocytes-macrophage cells during bacterial infections) is a more specific diagnostic marker of bacterial infection and values appear to correlate with severity.[41][42]​ Do not perform procalcitonin testing without an established, evidence-based protocol.[43]

Interleukins (IL-6, IL-32), as acute-phase reactants, are also being evaluated as possible markers to distinguish lower urinary tract infections from pyelonephritis.[44] Copeptin is a C-terminal part of pro-vasopressin (CT-pro-AVP) that is released along with vasopressin and has been investigated for use as a diagnostic tool in bacterial infections and sepsis.[40]

Imaging studies

Additional imaging is not usually necessary for diagnosis but can often be useful when patients are not responding to treatment as expected or after 72 hours.​[45] In patients with complicated infections, renal ultrasound may aid diagnosis by identifying hydronephrosis from a stone or other source of obstruction or show intra- or perirenal fluid collections and cysts.[46][47]

Contrast-enhanced spiral computed tomography (CECT) and/or magnetic resonance imaging of the abdomen can further delineate structural abnormalities to help guide therapy. Computed tomography of the abdomen can expose subjects to considerable radiation, but may be easier to schedule and is less expensive than magnetic resonance imaging.[45]

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