The patient who is most prone to the development of pyelonephritis is a patient with:

Radiology

Most uncomplicated cases of acute pyelonephritis do not require imaging studies unless symptoms do not improve, recurrence occurs, or the patient has prolonged fever (>72 hr) or persistent bacteremia. Abdominal radiographs (i.e., kidney, ureter, and bladder x-ray [KUB]) are of limited utility in acute pyelonephritis, unless staghorn calculi are present. Retrograde or antegrade pyelography may be helpful in severe obstruction that is not evident after noninvasive evaluations. Voiding cystourethrography demonstrates vesicoureteral reflux and generally conducted routinely only in children.

Recommendations for radiologic tests:

Healthy patients with uncomplicated pyelonephritis typically do not require radiologic evaluation when therapeutic responses occur within 72 hr of antibiotic therapy.

If no response to therapy occurs within 72 hr, abdominal CT is the study of choice.

Diabetics and immunocompromised patients should undergo precontrast and postcontrast abdominal and pelvic CT scans (Fig. 1) within 24 hr of diagnosis when response to therapy is not prompt.

Ultrasound (Fig. E2) is reserved for patients in whom exposure to contrast or radiation is considered hazardous. There is a high false-negative rate for renal abscess with ultrasound. In a prospective study of acute pyelonephritis of 213 patients submitted for CT/NMR study, 50 patients (23.5%) had a renal abscess, yet only two were detected by ultrasound.

All other adults with complicated cases (i.e., history of stones or other urologic conditions, prior urologic surgery, repeated episodes of pyelonephritis) should be evaluated early by CT.

Helical CT detects calculi with high sensitivity.

Urologic imaging studies should be conducted in all young men and boys.

FIG. E2. Acute pyelonephritis.

A, Subtle focal increased echogenic areas are seen in the anterior cortex of the right kidney.B, Single focal hypoechoic area is seen in the upper pole of the kidney in another patient.

Although the risk of contrast nephropathy has declined substantially, exert caution during contrast administration to patients with chronic kidney disease or for those taking metformin. When evaluating kidney function, diagnostic decision-making must include estimated glomerular filtration rate trends, not serum creatinine levels, especially in the elderly with reduced muscle mass. Patients with acute pyelonephritis and acutely elevated baseline serum creatinine concentrations may warrant CT imaging to rule out obstruction. If the risk of radiocontrast media administration outweighs its benefits, consider retrograde or antegrade pyelography.

Imaging

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Imaging work-up of UTI controversial

See professional society guidelines

With pyelonephritis, marked inflammatory response to renal parenchymal infection causes swelling that alters normal tissue properties & effectively ↓ radiologic contrast agent delivery to site, which results in

↓ uptake on nuclear cortical scan

↓ perfusion on Doppler imaging with altered echotexture on grayscale US

Striated or wedge-shaped foci of ↓ enhancement on CECT/MR

US with Doppler least invasive & readily available but less sensitive than nuclear renal cortical scans, CT, & MR

US frequently performed to search for associated complications (abscess, stones, scarring), congenital anomalies, & hydronephrosis

Urinary Tract Infection and Pyelonephritis

Pediatric UTIs constitute a significant health burden on society, affecting an estimated 2.6% to 3.4% of children yearly and resulting in more than 1 million physician visits annually (Freedman and Urologic Diseases in America, 2005). Throughout childhood, the risk of UTI is 2% for boys and 8% for girls. Although female infants have a two- to fourfold higher prevalence of UTI than male infants, the prevalence of UTI is highest in boys younger than 1 year of age (Shaikh et al., 2008). Among boys, an uncircumcised boy has a tenfold higher risk of UTI compared with a circumcised boy. In fact, circumcision is associated with decreased odds of UTI among boys with hydronephrosis (odds ratio 0.36, 95% confidence interval 0.29–0.44;Ellison et al., 2018). Given the association of UTI renal scarring that may lead to hypertension, proteinuria, and permanent renal damage, accurate diagnosis and treatment of UTI is important (Jakobsson et al., 1994).

Symptoms of UTI vary with patient age. Neither clinical symptoms nor laboratory tests will predict a UTI in febrile infants, and thus obtaining a urinary specimen is necessary (Crain and Gershel, 1990). Children younger than 2 years old often experience vague symptoms, including fever, irritability, poor feeding, vomiting, diarrhea, and ill appearance with increased likelihood of UTI in the setting of at least two of the following risk factors: (1) age less than 12 months, (2) white race, (3) absence of other fever source, (4) fever greater than 39°C, and (5) fever of 2 or more days (Subcommittee on Urinary Tract Infection, 2011). However, older children and adolescents may localize symptoms to the urinary tract, reporting dysuria, suprapubic pain, voiding dysfunction, incontinence, or flank and/or abdominal pain. There are no specific physical exam signs for UTI, although palpation in the suprapubic and flank areas may cause pain in the older child.

Pyelonephritis refers to an infection of the kidneys and is the most severe form of UTI in children. Clinically, this is associated with systemic features such as high fever, malaise, vomiting, abdominal and flank pain and tenderness, poor feeding, and irritability in infants (Strohmeier et al., 2014). Pyelonephritis is associated with significant short-term morbidity, including shock and septicemia, as well as acute kidney parenchymal injury. Permanent kidney damage may occur after acute pyelonephritis and is more frequent in children who have multiple episodes or who have vesicoureteral reflux.

Guidelines from the American Academy of Pediatrics and the National Institute for Health and Care Excellence recommend urine testing if treating a child for a suspected UTI (National Institute for Health and Clinical Excellence 2007,Subcommittee on Urinary Tract Infection, 2011).Specifically, a noncontaminated urine sample should be collected, which requires catheterization or suprapubic aspiration in infants and young children who have not completed toilet training. The diagnosis of UTI is generally confirmed by the pure growth of a bacteria of greater than 103 CFU/mL from a suprapubic aspirate or 104 CFU/mL from a bladder catheter specimen (Strohmeier et al., 2014).In addition, adolescents should be evaluated for sexually transmitted infections. Once a UTI is diagnosed, antimicrobial treatment should proceed based on local antibiograms. Importantly, even the most severe form of UTI in children, acute pyelonephritis, can be treated with oral antibiotics as long as the child can tolerate oral medication (Strohmeier et al., 2014).

DIAGNOSTIC CHECKLIST

Treatment

The first decision in the management of patients with APN is whether or not the patient requires hospitalization. Although prospective randomized trials are lacking, several retrospective studies as well as several prospective nonrandomized trials suggest that outpatient management is safe for many patients. Hospitalization should be considered for patients who cannot tolerate oral intake or who have severe pain or signs of severe sepsis. A strategy of initial management in the emergency department or an observation unit with an initial dose of parenteral antibiotic therapy, intravenous fluids, and symptomatic treatment of nausea and pain may be used in select patients to avoid hospital admission. Patients who will be treated as outpatients should have a stable social situation and the ability to contact the physician and return promptly if their symptoms worsen. Hospitalization is generally recommended for patients with complicated infections. Most experts believe that pregnant women with APN should always be hospitalized.

There are surprisingly few prospective randomized trials of the treatment of pyelonephritis. For patients who require hospitalization, parenteral therapy with an aminoglycoside, a third-generation cephalosporin, or a fluoroquinolone is recommended. At my institution we discourage fluoroquinolones for this indication because there are other effective alternatives and we wish to minimize the use of these very broad-spectrum agents in the hospital setting. Although resistance to TMP-SMX among uropathogenicE. coli appears to have leveled off and might actually be decreasing, this agent should not be used for empiric therapy of APN. If a ESBL-producing organism is suspected, empiric therapy should include a carbapenem. Because risk factors for ESBL-producing organisms are not well defined, consideration for coverage of these pathogens should be given in all patients with APN who have severe sepsis or septic shock. Multi-drug resistant pathogens may require treatment with newer B-lactam/B-lactamase inhibitor drugs such as ceftazidime-avibactam (Avycaz), ceftolozane-tazobactam (Zerbaxa), or meropenem-vaborbactam (Vabomere).

If a gram-positive pathogen is suspected or suggested by the results of urine Gram stain, ampicillin or ampicillin-sulbactam (Unasyn) with or without an aminoglycoside can be used. Patients should receive intravenous therapy until they are clinically improving and able to reliably tolerate oral intake; oral therapy can be chosen based on the results of urine culture and susceptibility data. TMP-SMX, a fluoroquinolone, and ampicillin are all potential candidates for oral switch therapy. The narrowest spectrum, least expensive agent to which the isolated pathogen is susceptible should be chosen. Despite in vitro susceptibility data, first- and second-generation cephalosporins have a poor track record in the treatment of APN and are generally not recommended, with the exception of pyelonephritis in pregnancy.

EPIDEMIOLOGY

Annually in the USA, approximately 200 000 adults are admitted to hospital for renal infection;1,2 many others are managed as outpatients. In Seattle, Washington, the annual risk of pyelonephritis is approximately 12–13/10 000 for women and 3–4/10 000 for men.3

Imaging Findings

Uncomplicated pyelonephritis may demonstrate an enlarged, swollen kidney with areas of increased or decreased renal parenchymal echogenicity. More so in children, power and color Doppler techniques have been used to diagnose pyelonephritis based on focal areas of hypovascularity.

On CT, simple pyelonephritis may demonstrate a striated nephrogram as well as ill-defined hypoattenuating areas within the parenchyma. This is caused by the decreased excretory function of the renal tubules secondary to obstruction by inflammatory debris, vasospasm, and surrounding parenchymal edema. Delayed enhancement secondary to vasospasm may be identified on the affected side. Other findings associated with pyelonephritis on CT may be thickening and hyperenhancement of the urothelium. Also, perinephric stranding may be identified.

Acute Pyelonephritis

Pyelonephritis is a potentially devastating complication of transplantation. Pyelonephritis can present as acute renal failure406,407 and cause graft loss.408,409 According to one series, pyelonephritis arises most often 1 year or more after transplantation (80% of episodes), and E. coli was the most common organism (80%).410 Acute pyelonephritis is a not an uncommon finding on renal biopsy, despite the expectation that the process is patchy.406 Renal biopsies are not the usual method of diagnosis, however, if neutrophils are abundant, especially if they form destructive abscesses and casts in tubules, the diagnosis should be at the top of the list. Other variants are emphysematous pyelonephritis, due to gas-producing organisms,409 xanthogranulomatous pyelonephritis,411,412 and malakoplakia.413

Pyelonephritis

Pyelonephritis is defined as infection and inflammation of the kidney and renal pelvis. Its diagnosis is clinical, and symptoms include back or flank pain with costovertebral angle tenderness on examination, fever (temperature higher than 38 °C), bacteriuria, and possibly nausea and vomiting. Dysuria is a less common presenting symptom for pyelonephritis. E. coli accounts for up to 80% of cases. Other causative organisms include species of Klebsiella, Proteus, Enterobacter, Pseudomonas, Serratia, and Citrobacter species.

All patients with suspected pyelonephritis require a urinalysis and culture with sensitivity tests. Pyuria is almost universal in pyelonephritis, and white cell casts on urinalysis indicate upper urinary tract involvement. Infected patients can have an elevated white blood cell count, erythrocyte sedimentation rate, and C‐reactive protein. Imaging is not usually required to make the diagnosis of pyelonephritis.

In patients with a mild infection without fever or other systemic complications, outpatient therapy is appropriate. All other patients should be hospitalized, particularly any patient who fits criteria for a complicated infection (e.g., immunocompromised patients, patients with urinary tract anomalies or infection associated with urologic surgery) or who may not be compliant. All pregnant patients with pyelonephritis should be hospitalized due to the risk of escalating infection, preterm labor, and possible sepsis with acute respiratory distress syndrome.

Recommended outpatient therapy includes a fluoroquinolone for 7 days or TMP‐SMX for 14 days if the organism is known to be susceptible. For a gram‐positive infection, amoxicillin or amoxicillin/clavulanic acid (Augmentin) can be used alone. Treatment can be refined once antibiotic sensitivities are available.

For inpatient therapy, parenteral therapy is recommended. This may consist of a fluoroquinolone, an aminoglycoside such as gentamicin with or without ampicillin, or an extended‐spectrum cephalosporin with or without an aminoglycoside. After improvement and resolution of fever, the patient can be discharged to home with orders to complete 2 weeks of therapy. Ideally, culture‐proven sensitivities can be used to design this regimen. For all treatment regimens, CT imaging or renal ultrasonography should be considered to rule out perinephric or intrarenal abscesses if no improvement of symptoms is seen within 72 hours.

PYELONEPHRITIS DURING PREGNANCY

Pyelonephritis during pregnancy is traditionally treated with hospitalization and intravenous antibiotics. The ACOG recommends inpatient intravenous therapy until the patient is afebrile and symptomatically improved, followed by outpatient oral antibiotics to complete 10 days of therapy.37 Several studies have been done to explore the safety of outpatient pyelonephritis treatment in pregnant women. In one study, 120 women with a gestational age less than 24 weeks were randomized to receive intramuscular ceftriaxone as outpatients or intravenous cefazolin as inpatients.43 Those to be treated as outpatients were observed for up to 24 hours or until clinically stable and were seen by home health nurses between 18 and 36 hours after discharge for a second intramuscular dose of antibiotics, followed by a course of oral cephalexin. Ten percent of the outpatients were hospitalized because of sepsis, recurrent pyelonephritis, or abnormal laboratory test results. Eleven outpatients (18%) and 12 inpatients (20%) had positive urine cultures at follow-up (RR = 0.9; 95% CI: 0.4 to 1.9; P = .82). No significant differences between the groups were found in rates of recurrent pyelonephritis, abortion, or preterm delivery. There were no serious complications or pregnancy losses from either treatment regimen.35,42

Another trial randomized women with pyelonephritis at a gestational age greater than 24 weeks to receive two doses of intramuscular ceftriaxone, followed by oral cephalexin in an inpatient or outpatient setting.44 Those randomized to outpatient care were arranged to be discharged after 24 hours of observation if clinically stable, although almost 30% of those women remained as inpatients due to sepsis, preterm labor, or other medical complications. No significant differences were seen between the inpatient and outpatient groups in the numbers of repeat positive urine culture after treatment (P = .44), recurrent pyelonephritis (P = .35), or preterm delivery (P = .75). More than 60% (154 of 246) of the women initially eligible for the study were eliminated due to preterm labor, obvious sepsis, recurrent pyelonephritis, or other preexisting medical conditions. The evidence suggests that although it may be effective to treat some patients at a gestational age greater than 24 weeks as outpatients, only very select groups may be safely treated in this manner.35,44 ACOG comments on the limited evidence in this area and emphasizes that if these patients are to be treated as outpatients, home health care should be provided.37

The ACOG states that all women treated for urinary tract infections should be rescreened periodically for infection with urine dipstick tests or cultures. If the infection recurs, the patient should be retreated, and chronic suppression should be considered.37