Investigations
1st investigations to order
urinalysis
Test
Obtain a midstream, clean-catch urine sample. Perform a dipstick for urinalysis routinely in all patients with suspected acute pyelonephritis to confirm the diagnosis.[34] Assess for white blood cells, red blood cells, and nitrites.[34]
Note that Public Health England guidance differs from European guidelines; it advises not to perform urine dipsticks in patients over 65 years of age as the results become more unreliable with age.[27]
Practical tip
A urine dipstick test looks for positive leukocytes or nitrites to indicate a urinary tract infection (UTI).[3] Organisms such as Escherichia coli or Klebsiella, Enterobacter, Proteus, Staphylococcus, or Pseudomonas species reduce nitrate to nitrite in the urine; therefore, the presence of nitrite on a urinalysis is an indicator of a UTI.[41]
urine culture and sensitivity
Test
Obtain a midstream, clean-catch urine sample to send for culture, microscopy, and susceptibility testing before antibiotics are taken.[33][34][36]
Antimicrobial susceptibility testing will help to inform definitive treatment.[27][34]
Over 95% of women with uncomplicated acute pyelonephritis will have >100,000 colony-forming units (CFU)/mL of a single gram-negative organism.[33]
Advise the patient on how to collect the sample:[27]
Women catching midstream urine samples can try to part the labia if possible to help reduce contamination.[27] Men should pull back the foreskin
Recommend against cleaning the area with antiseptic as this can inhibit bacteria[27]
Catch a sample in a sterile container in an older patient with incontinence[27]
In a patient with a catheter, collect from the sampling port using an aseptic technique.[27]
Consider sending a repeat sample if mixed growth or epithelial cells are present in a patient with symptoms of a lower urinary tract infection.[27] Consider a repeat sample after 2 to 3 days of therapy if symptoms do not improve, or if symptoms recur within 2 weeks of treatment.[33]
Consider the presenting signs and symptoms when interpreting the result.[27]
Result
bacteria ≥100,000 CFU/mL[33]
full blood count
Test
Check for leukocytosis, which is present in acute infection.
Result
leukocytosis
erythrocyte sedimentation rate
Test
A non-specific marker of infection.
Result
elevated
C-reactive protein
Test
A non-specific marker of infection.
Result
elevated
urea and electrolytes
Test
Request to assess kidney function.
Result
deranged
creatinine
Test
Request to assess kidney function.
Result
elevated with impaired kidney function
blood culture (in hospital)
Test
Request a blood culture in a patient requiring admission to hospital.[33] Do not request routinely in a patient with uncomplicated pyelonephritis.
Draw blood before administering antibiotics.
Check for the presence of any bacterial growth.
Between 15% and 30% of people with acute pyelonephritis are found to be bacteraemic on blood culture.[33]
Result
presence of bacterial growth
Investigations to consider
renal ultrasound
Test
Use ultrasound to rule out urinary tract obstruction or renal stone disease in patients with a history of urolithiasis, renal function disturbances, or a high urine pH.[34]
In pregnant women, use an ultrasound scan preferentially to avoid radiation risk to the fetus when diagnosing complications.[34]
Result
stones; structural abnormalities
contrast-enhanced CT
Test
Consider contrast-enhanced CT if a patient remains febrile after 72 hours of treatment, or immediately if their clinical status deteriorates.[34]
Consider the risks of contrast-induced acute kidney injury in a patient with reduced kidney function.[33]
Use CT in a patient where there is still clinical suspicion of renal stones, even if not shown on ultrasound.
Result
focal wedge-like regions of the kidney, which appear swollen and demonstrate reduced enhancement compared with the normal sections of the kidney
MRI
Test
In pregnant women, use MRI preferentially to avoid radiation risk to the fetus when diagnosing complications.[34]
Result
can be normal with mild infection; with severe infection, parts of the kidney may appear oedematous and of lower attenuation; other findings may include renal calculi or gas within the collecting system; perinephric stranding can be seen
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