ACR Appropriateness Criteria®
Clinical Condition: Acute Pyelonephritis
Variant 1: Uncomplicated patient.
| Radiologic Procedure |
Rating |
Comments |
RRL* |
| X-ray intravenous urography |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. |
Med |
| X-ray voiding cystourethrography |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. |
Low |
| CT abdomen and pelvis without and with contrast |
1 |
If there is a role for imaging in some circumstance, this is most likely to provide the most information. |
High |
| NUC Tc-99m DMSA scan kidney |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. |
Med |
| X-ray abdomen (KUB) |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. |
Med |
| US kidneys and bladder retroperitoneal |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. |
None |
| X-ray antegrade pyelography |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. |
Med |
| MRI abdomen and pelvis without and with contrast |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. |
None |
| CT abdomen and pelvis without contrast |
1 |
Studies show that imaging adds little to management if the patient responds to therapy within 72 hours. May be used in the presence of a critical diagnosis where contrast material cannot be given. |
High |
| Rating Scale: 1=Least appropriate, 9=Most appropriate |
*Relative Radiation Level |
Note: Abbreviations used in the tables are listed at the end of the "Major Recommendations" field.
Variant 2: Complicated patient (e.g., diabetes, immunocompromised, history of stones, prior renal surgery, not responding to therapy).
| Radiologic Procedure |
Rating |
Comments |
RRL* |
| CT abdomen and pelvis without and with contrast |
8 |
|
High |
| US kidneys and bladder retroperitoneal with KUB |
6 |
May be used as an alternative study to above. |
Med |
| CT abdomen and pelvis without contrast |
5 |
|
High |
| MRI abdomen and pelvis without and with contrast |
4 |
See comments regarding contrast in the text below under "Anticipated Exceptions." |
None |
| X-ray voiding cystourethrography |
3 |
Not part of initial evaluation but may be used subsequently to demonstrate clinically suspected reflux. |
Low |
| NUC Tc-99m DMSA scan kidney |
3 |
Cannot differentiate renal parenchymal disease from perinephric process. |
Med |
| X-ray abdomen (KUB) |
2 |
Insufficient information by itself to guide therapy. |
Med |
| X-ray intravenous urography |
2 |
|
Med |
| X-ray antegrade pyelography |
1 |
Not an initial study. |
Med |
| Rating Scale: 1=Least appropriate, 9=Most appropriate |
*Relative Radiation Level |
Note: Abbreviations used in the tables are listed at the end of the "Major Recommendations" field.
Summary of Literature Review
Urinary tract infections are among the most common infections affecting humans. In most adults, the infection is confined to the lower urinary tract (LUT), the diagnosis is established by clinical or laboratory studies, and imaging studies are not required. When the kidney itself is involved or when there is difficulty in differentiating LUT infection from renal parenchymal involvement, imaging studies are often requested both for diagnosis and to plan management. Conditions that are thought to predispose a patient with LUT infection to renal involvement include vesicoureteral reflux, altered bladder function, congenital urinary tract anomalies, and the presence of renal calculi.
Pathologically, inflammatory disease of the kidney generally occurs as the result of ascending infection from the LUT (whether or not radiologically demonstrated vesicoureteral reflux is present) by gram-negative enteric pathogens (usually Escherichia coli) and is known as acute pyelonephritis. This name accurately reflects the underlying pathologic condition present (i.e., infection involving both the renal parenchyma and the renal pelvis). In the majority of patients, uncomplicated pyelonephritis is readily diagnosed clinically and responds quickly to treatment with appropriate antibiotics. If the treatment is started late, the patient is immunocompromised, or, for other poorly understood reasons, small microabscesses that form during the acute phase of pyelonephritis may coalesce to form an acute renal abscess. If such an abscess then ruptures into the perinephric space, a perirenal abscess is formed. If the infection is confined to an obstructed collecting system, the infection is referred to as pyelonephrosis. Patients with underlying diabetes are of particular concern. Not only are they more vulnerable to the development of a complication from acute pyelonephritis, but it is also more difficult to establish the diagnosis on clinical grounds in diabetics, since as many as 50% will not have the typical flank tenderness that helps to differentiate pyelonephritis from LUT infection in an otherwise healthy patient.
Prior to the advent of cross-sectional imaging, radiologic studies performed in patients with uncomplicated pyelonephritis were normal in most cases. In the early 1970s, however, a subgroup of patients was identified with acute pyelonephritis, commonly with underlying diabetes, who did not respond quickly to therapy and in whom urography showed anatomic and severe functional abnormalities. In order to differentiate such patients from those with garden-variety pyelonephritis, a new term, acute bacterial nephritis, was coined. With the advent of cross-sectional imaging, a whole new lexicon of terminology evolved to describe various degrees of parenchymal involvement with pyelonephritis. The Society of Uroradiology has recommended that all patients with renal infection be referred to as having acute pyelonephritis, with only the additional modifiers unilateral or bilateral, focal or diffuse, focal swelling or no focal swelling, and renal enlargement or no enlargement used to describe the extent of the process.
Intravenous Urography
Traditionally, intravenous urography (IVU) has been the primary diagnostic modality for imaging patients with renal infection. The rationale for performing urography is not to diagnose acute pyelonephritis but to look for an underlying anatomic abnormality (i.e., anomaly) that may have predisposed the patient to the infection; to search for a process such as a calculus, papillary necrosis, or obstruction that may prevent a rapid therapeutic response; or to diagnose a complication of the infection such as a renal or perinephric abscess. As such, many urologists routinely order an IVU or, with increasing frequency, computed tomography (CT) of the abdomen and pelvis in all patients with clinical pyelonephritis within the first 24 hours after initiation of therapy.
There is now reasonably good evidence that routine urography does not alter the clinical care in 90% of patients with pyelonephritis. This same study showed, however, that if investigation was confined to those patients who did not become afebrile after 72 hours of appropriate antibiotics therapy, the number of patients with urographic findings of immediate clinical significance rose to 36%. The authors also found a five-fold increase in yield from routine urography in patients with underlying diabetes or those infected with a pathogen other than ampicillin-sensitive Escherichia coli. Other authors confirmed the validity of the 72-hour period in a study of the utility of CT in patients with pyelonephritis; in this series, 95% of patients with uncomplicated pyelonephritis became afebrile within 48 hours of appropriate antibiotic therapy, and nearly 100% did so within 72 hours.
Computed Tomography and Ultrasound (US)
There is almost universal agreement that precontrast and postcontrast CT is the imaging study of choice to diagnose patients with atypical pyelonephritis or to look for a potential complication of the infection such as a renal or perinephric abscess or a renal emphysema. In most of the studies comparing CT with US, much of the superiority of CT lay in its ability to detect parenchymal abnormalities in patients with pyelonephritis that are generally missed by US but do not alter the patient´s therapy. One study, however, reported that US missed 6 of 10 intrarenal and 1 of 5 perinephric abscesses subsequently diagnosed by CT. In only three of these cases, however, were the results verified by surgery. The proponents of US are quick to point out its advantages -- namely, low risk, relatively low expense, lack of ionizing radiation, and, most importantly, the fact that it does not require the use of contrast material. Recent technical advances in US such as tissue harmonic imaging and the use of US contrast agents have been shown to increase the sensitivity of US to subtle parenchymal abnormalities in pyelonephritis, but further work in this area is needed before definite recommendations can be made. Conventional gray-scale US has been considered the method of choice to diagnose pyelonephrosis (i.e., low-level echoes within the collecting system), but CT can also suggest this diagnosis. The most specific test to diagnose pyelonephrosis, however, is needle aspiration of the collecting system, which is generally performed as a prelude to percutaneous nephrostomy.
Other Imaging Studies
Recently there has been increased interest in the diagnosis of acute pyelonephritis using technetium 99m dimercaptosuccinic acid (DMSA) renal scintigraphy, particularly in children. Recent studies have shown this technique to be much more sensitive for detecting pyelonephritis than US. Recently, Power Doppler US has shown sensitivities and specificities approaching 90% in children with acute pyelonephritis. This is important in children since differentiating LUT infection from pyelonephritis is more difficult in the pediatric population and since it is the young who are more vulnerable to permanent renal damage from renal inflammatory disease. One recent study, however, suggests that CT is more accurate than technetium 99m DMSA renal scintigraphy in detecting acute pyelonephritis lesions in adults.
Various other imaging studies are of value in selected patients. Magnetic resonance imaging (MRI) is felt to be useful in patients in whom the use of iodinated contrast material must be avoided, (i.e., those with azotemia or contrast sensitivity), but case-controlled studies documenting its efficacy have yet to be published. While gadolinium-based contrast agents enhanced inversion recovery MRI has been shown to be only slightly less sensitive and specific than DMSA scintigraphy for acute pyelonephritis in children. One potential disadvantage of MRI is its inability to detect smaller calculi. Retrograde pyelography is of value in patients with severe infection and obstruction that cannot be demonstrated noninvasively. Antegrade pyelography can be used as an alternative to the retrograde study. Voiding cystourethrography is used to demonstrate vesicoureteral reflux, but is generally only routinely performed in children.
Summary
Otherwise healthy patients with uncomplicated pyelonephritis probably need no radiologic work-up if they respond to antibiotic therapy within 72 hours. If there is no response to therapy, CT of the abdomen and pelvis is the study of choice. Diabetics or other immunocompromised patients should probably be evaluated with precontrast and postcontrast CT within 24 hours of diagnosis if response is not prompt. US should be reserved for patients in whom pyelonephrosis is suspected and those patients for whom exposure to contrast or radiation is hazardous. All other complicated adult patients (e.g., patients with a history of stones or other urologic conditions, prior urologic surgery, repeated episodes of pyelonephritis) probably deserve early evaluation with CT.
Anticipated Exceptions
Nephrogenic systemic fibrosis (NSF, also known as nephrogenic fibrosing dermopathy) was first identified in 1997 and has recently generated substantial concern among radiologists, referring doctors and lay people. Until the last few years, gadolinium-based MR contrast agents were widely believed to be almost universally well tolerated, extremely safe and non-nephrotoxic, even when used in patients with impaired renal function. All available experience suggests that these agents remain generally very safe, but recently some patients with renal failure who have been exposed to gadolinium contrast agents (the percentage is unclear) have developed NSF, a syndrome that can be fatal. Further studies are necessary to determine what the exact relationships are between gadolinium-containing contrast agents, their specific components and stoichiometry, patient renal function and NSF. Current theory links the development of NSF to the administration of relatively high doses (e.g., >0.2 mM/kg) and to agents in which the gadolinium is least strongly chelated. The U.S. Food and Drug Administration (FDA) has recently issued a "black box" warning concerning these contrast agents (http://www.fda.gov/cder/drug/InfoSheets/HCP/gcca_200705HCP.pdf).
This warning recommends that, until further information is available, gadolinium contrast agents should not be administered to patients with either acute or significant chronic kidney disease (estimated glomerular filtration rate [GFR] <30 mL/min/1.73m2), recent liver or kidney transplant or hepato-renal syndrome, unless a risk-benefit assessment suggests that the benefit of administration in the particular patient clearly outweighs the potential risk(s).
Abbreviations
- CT, computed tomography
- DMSA, dimercaptosuccinic acid
- KUB, kidneys, ureters, bladder
- Med, medium
- MRI, magnetic resonance image
- NUC, nuclear medicine
- Tc, technetium
- US, ultrasound
| Relative Radiation Level |
Effective Dose Estimated Range |
| None |
0 |
| Minimal |
<0.1 mSv |
| Low |
0.1-1 mSv |
| Medium |
1-10 mSv |
| High |
10-100 mSv |
