patients with pyelonephritis often have chills that start suddenly , fever , and pain between the ribs and hips on one or both sides of the body . based on these symptoms
computed tomography ( ct ) , which allows doctors to look inside the body , is useful for letting doctors determine whether something is blocking the urinary tract .
this type of imaging is often used for diagnosing pyelonephritis ; some doctors believe that a condition called perirenal fat stranding ( pfs ) , in which swelling of the fat around the kidneys is visualized on ct , is a characteristic finding of pyelonephritis . however , it is not clear whether this belief is true .
we found that pfs was not specific to pyelonephritis , ie , it could be associated with other conditions .
we also found that ct was not the most cost - effective way to diagnose pyelonephritis , as patients symptoms and laboratory parameters were more relevant diagnostic tools .
we believe that our study is important as it evaluated pfs with respect to pyelonephritis .
urinary tract infections ( utis ) are very common . in the us , they account for ~8.1 million visits to health care providers annually.1 utis are termed differently depending on the part of the body affected ; an infection of the kidney / upper urinary tract is called pyelonephritis , while an infection of the urethra is called urethritis . among these infections , pyelonephritis can be complicated by sepsis , and in many cases , hospitalization is necessary for treatment.2 classical symptoms of pyelonephritis include an abrupt onset of chills , fever , and unilateral or bilateral flank pain with costovertebral angle ( cva ) tenderness.3 in addition to these physical findings , a combination of urine analysis ( via microscopy and/or reagent strip ) , urine cultures , and blood cultures may be used to diagnose pyelonephritis .
however , the diagnosis of acute pyelonephritis can be difficult for clinicians , especially among elderly patients who present with unusual symptoms , such as abdominal pain , vomiting , and diarrhea .
computed tomography ( ct ) is a useful diagnostic tool for providing comprehensive anatomical and physiological information that accurately characterizes both intra- and extrarenal pathological conditions.3 imaging with ct is not unusual in the diagnosis of acute pyelonephritis and is often used for patients who do not respond to antibiotics within 72 hours or patients with underlying conditions , such as pregnancy , diabetes mellitus , and immunocompromised state , among others.4,5 swelling of the kidney , hypoenhanced renal parenchyma , and pronounced perirenal fat stranding ( pfs ) are some of the ct findings that may accompany acute pyelonephritis.6,7 yu et al8 have reported that perinephric fat stranding was found in 29.1% of patients with acute pyelonephritis .
some clinicians regard pfs as a characteristic finding of pyelonephritis ( figure 1).9 however , to the best of our knowledge , no study has reported on the factors that are associated with pfs as well as the sensitivity and/or specificity of pfs in diagnosing acute pyelonephritis .
thus , we aimed to evaluate the correlation between age , renal function , and frequency of pfs .
this retrospective observational study was conducted at the japanese red cross ishinomaki hospital , ishinomaki , miyagi , japan .
this study was approved by the ethical committee of japanese red cross ishinomaki hospital ( approval no 16 - 28 ) and was conducted in accordance with the 1964 declaration of helsinki and its later amendments or comparable ethical standards .
the ethics committee deemed written informed consent not necessary due to the retrospective nature of the study .
the medical records of all patients who were admitted to the japanese red cross ishinomaki hospital in the miyagi prefecture of japan between december 2013 and july 2015 were reviewed .
of 468,624 patients , we selected 120 patients with the 10th revision of the international statistical classification of diseases and related health problems ( icd-10 ) code n10 in their medical records .
the diagnosis of acute pyelonephritis was based on physical examination ( presence of fever , chills , and cva tenderness ) and laboratory findings ( increased white blood cell [ wbc ] count , elevated c - reactive protein [ crp ] , and demonstration of pyuria and bacteriuria on urine testing ) .
we excluded eleven cases of children ( aged < 16 years ) and two cases that did not demonstrate clear findings of acute pyelonephritis .
of the remaining cases , 89 had undergone ct evaluation of both kidneys ; we evaluated these patients for the presence or absence of pfs as the pyelonephritis group .
these patients underwent percutaneous renal biopsy at our hospital between january 2013 and may 2016 ; each of them had undergone ct examination the day after the renal biopsy to check for bleeding complications.10 we had ruled out acute pyelonephritis prior to percutaneous renal biopsy in this group , given that acute pyelonephritis is a contraindication for renal biopsy . in this control group ,
ct images were obtained using an aquilion cxl system ( toshiba medical systems corp . , tochigi , japan ) .
normally distributed data are presented as mean  standard deviation ( sd ) , with nonparametric data presented as the median and 25th75th percentile .
statistical analyses were performed using the student s t - test for parametric data , the mann 
whitney u test for nonparametric data , fisher s exact test for nominal categorical valuables , and logistic regression analysis for the following covariates : age , sex , and estimated glomerular filtration rate ( egfr ) .
we performed 1:1 matching based on the propensity score , as patient characteristics of both the pyelonephritis and control groups were different.11,12 the purpose of using the propensity score was to reduce the influence of bias by aligning the backgrounds of both groups . in this study ,
logistic regression was performed with the presence or absence of pyelonephritis as a dependent variable , and age , sex , and renal function as independent variables . a propensity score for whether or not pyelonephritis develops from a background factor was prepared .
all statistical data were analyzed using ezr version 3.3.1 ( saitama medical center , jichi medical university , saitama , japan ) , which is a graphical user interface for r ( the r foundation for statistical computing , vienna , austria).13
this retrospective observational study was conducted at the japanese red cross ishinomaki hospital , ishinomaki , miyagi , japan .
this study was approved by the ethical committee of japanese red cross ishinomaki hospital ( approval no 16 - 28 ) and was conducted in accordance with the 1964 declaration of helsinki and its later amendments or comparable ethical standards .
the ethics committee deemed written informed consent not necessary due to the retrospective nature of the study .
the medical records of all patients who were admitted to the japanese red cross ishinomaki hospital in the miyagi prefecture of japan between december 2013 and july 2015 were reviewed .
of 468,624 patients , we selected 120 patients with the 10th revision of the international statistical classification of diseases and related health problems ( icd-10 ) code n10 in their medical records .
the diagnosis of acute pyelonephritis was based on physical examination ( presence of fever , chills , and cva tenderness ) and laboratory findings ( increased white blood cell [ wbc ] count , elevated c - reactive protein [ crp ] , and demonstration of pyuria and bacteriuria on urine testing ) .
we excluded eleven cases of children ( aged < 16 years ) and two cases that did not demonstrate clear findings of acute pyelonephritis .
of the remaining cases , 89 had undergone ct evaluation of both kidneys ; we evaluated these patients for the presence or absence of pfs as the pyelonephritis group .
these patients underwent percutaneous renal biopsy at our hospital between january 2013 and may 2016 ; each of them had undergone ct examination the day after the renal biopsy to check for bleeding complications.10 we had ruled out acute pyelonephritis prior to percutaneous renal biopsy in this group , given that acute pyelonephritis is a contraindication for renal biopsy . in this control group ,
normally distributed data are presented as mean  standard deviation ( sd ) , with nonparametric data presented as the median and 25th75th percentile .
statistical analyses were performed using the student s t - test for parametric data , the mann 
whitney u test for nonparametric data , fisher s exact test for nominal categorical valuables , and logistic regression analysis for the following covariates : age , sex , and estimated glomerular filtration rate ( egfr ) .
we performed 1:1 matching based on the propensity score , as patient characteristics of both the pyelonephritis and control groups were different.11,12 the purpose of using the propensity score was to reduce the influence of bias by aligning the backgrounds of both groups . in this study ,
logistic regression was performed with the presence or absence of pyelonephritis as a dependent variable , and age , sex , and renal function as independent variables . a propensity score for whether or not pyelonephritis develops from a background factor was prepared .
all statistical data were analyzed using ezr version 3.3.1 ( saitama medical center , jichi medical university , saitama , japan ) , which is a graphical user interface for r ( the r foundation for statistical computing , vienna , austria).13
the characteristics of the enrolled patients are presented in table 1 . compared with patients in the control group ,
patients in the pyelonephritis group were significantly older ( 7415 years vs 6316 years , respectively , p<0.001 ) and had a significantly higher frequency of pfs ( 72% vs 39% , respectively , p<0.01 ) .
other findings are as follows : 25 ( 28% ) and eight ( 9.1% ) patients in the pyelonephritis group had underlying diabetes and a history of immunosuppressant use compared to 195 ( 61% ) and 62 ( 20% ) patients , respectively , in the control group .
a total of 26 ( 52% ) patients in the pyelonephritis group had cva tenderness . in terms of serological findings ,
the median wbc count was 11,300/l ( interquartile range ( iqr ) : 8,10017,600/l ) in the pyelonephritis group vs 6,100/l ( iqr : 4,1007,700/l ) in the control group , while the median crp was 12 mg / dl ( iqr : 4.718 mg / dl ) in the pyelonephritis group vs 0.13 mg / dl ( iqr : 0.050.56 mg / dl ) in the control group .
the median lactate level was 1.6 in the pyelonephritis group ( lactate was not measured in the control group ) . in terms of urinary findings , the positive rate of wbc was 83% ( 70/85 ) in the pyelonephritis group vs 20% ( 63/318 ) in the control group , that of nitrite was 37% ( 31/85 ) in the pyelonephritis group vs 4.4% ( 14/318 ) in the control group , and that of wbc or / and nitrite was 93% ( 79/85 ) in the pyelonephritis group vs 21% ( 67/318 ) in the control group .
bacteria were identified in urine cultures in 92% ( 71/77 ) of patients in the pyelonephritis group .
ct imaging detected urinary stones in 40% ( 36/89 ) of patients in the pyelonephritis group vs 12% ( 39/319 ) of patients in the control group , hydronephrosis in 47% ( 42/89 ) of patients in the pyelonephritis group vs 3.8% ( 12/319 ) of patients in the control group , and renal emphysema in 3.4% ( 3/89 ) of patients in the pyelonephritis group vs 0.31% ( 1/319 ) of patients in the control group . among the 89 patients in the pyelonephritis group , 51
were subjected to contrast ct ; of these , 22 ( 43% ) had areas where renal contrast was insufficient .
for all enrolled patients , the sensitivity , specificity , and positive likelihood ratio of pfs in diagnosing acute pyelonephritis were 72% ( 95% confidence interval [ ci ] , 6181 ) , 71% ( 95% ci , 6676 ) , and 2.5 ( 95% ci , 2.03.1 ) , respectively ( table 2 ) .
based on our empirical speculation that pfs was affected by age , sex , and renal function , we divided patients into two groups for each of the variables : age ( younger vs older ) , sex ( male vs female ) , and renal function ( high vs low ) ; the frequency of pfs was determined for each group ( figures 35 ) . in the pyelonephritis group ,
( aged < 70 years ) vs 73% in older patients ( aged 70 years ) , 84% in men vs 67% in women , and 80% in those with egfr < 35 ml / min/1.73 m vs 64% in those with egfr > 35 ml / min/1.73 m. no significant differences were found between any of these groups . in the control group ,
the frequency of pfs was 29% in younger patients vs 54% in older patients , 47% in men vs 28% in women , and 51% in those with egfr < 35 ml / min/1.73 m vs 28% in those with egfr > 35 ml / min/1.73 m. the reason for dividing egfr into more or less than 35 ml / min/1.73 m was because the median value of both the pyelonephritis and control groups was ~35 ml / min/1.73 m. the kidney disease : improving global outcomes guidelines classify the severity of chronic kidney disease by gfr.14 gfr 3044 is classified as g3b category ( moderately to severely decreased ) , and gfr 1530 is classified as g4 category ( severely decreased ) .
therefore , the 35 cutoff values used in this study were considered to be reasonable in general , with no significant divergence from the severity classification by gfr .
a multiple logistic regression model yielding odds ratios ( ors ) and 95% cis was then used to identify predictors of pyelonephritis .
the model included age , sex , and egfr . in the multiple logistic regression model ,
the or for pfs to non - pfs was 5.0 ( 95% ci , 2.79.3 ; p<0.01 ) .
next , we performed a post hoc 1:1 person match by assigning the two groups ( pyelonephritis group and control group ) to each case , matched for age , sex , and egfr on admission ( table 3 ) .
conditional logistic regression analyses for these propensity score - matched data sets were used to estimate the ors and respective 95% cis.7 the or for pfs to non - pfs was 3.6 ( 95% ci , 1.96.7 ; p<0.01 ) . after adjusting for age , sex , and renal function using the propensity score analysis , the sensitivity , specificity , and positive likelihood ratio of pfs for diagnosing acute pyelonephritis was 72% ( 95% ci , 6181 ) , 58% ( 95% ci , 4869 ) , 1.7 ( 95% ci , 1.32.3 ) , respectively ( table 4 ) . using the mantel 
haenszel test , the or for pfs to non - pfs was found to be 2.9 ( 95% ci , 1.66.5 ; p<0.01 ) .
this study revealed that pfs is affected by age , sex , and egfr , with increasing age , male sex , and lower egfr increasing the frequency of pfs .
however , the presence of pfs itself was not a useful indicator of acute pyelonephritis . as shown in table 1 ,
pfs was found in 39% of patients , which seemed high compared to 29.1% reported by yu et al .
however , in their study , the mean age of enrolled patients was 6017 years and 93% were females.8 in contrast , the mean age of enrolled patients in our study was 7415 years and only 72% were females . considering that older age and male sex increase the frequency of pfs , we believe that our findings were acceptable .
previous reports did not include a control group , making it impossible to calculate the sensitivity and specificity . in our study
, we included patients who had undergone renal biopsy as our control group , after acute pyelonephritis had been ruled out by the treating physician ( given that active uti is a contraindication for percutaneous renal biopsy ) .
this allows us to calculate the sensitivity and specificity . in the control group , 67 ( 21% )
patients had positivity of wbc and/or nitrite in urinalysis ; however , all of them had asymptomatic bacteriuria .
there were ~20% of patients with asymptomatic bacteriuria when the control group was compared to the whole population ; it might be related to the fact that the average age was higher and there were more diabetic patients and immunosuppressant users .
although hypoenhanced regions in the kidney may be characteristic of acute pyelonephritis , they are not specific . for example , acute renal infarction and immunoglobulin g4-related renal disease ( igg4rd ) are accompanied by hypoenhanced regions in the kidney.15,16 in addition , acute renal infarction is also associated with fever , flank pain , and increased inflammatory markers , similar to acute pyelonephritis .
however , elevated serum lactate dehydrogenase is found in almost all patients with acute renal infarction , and this may be useful in distinguishing acute renal infarction from acute pyelonephritis .
these findings suggest that radiological imaging is not as useful as conventional examination in the diagnosis of acute pyelonephritis .
the sensitivity of urinalysis ( > 10 wbcs / high power field [ hpf ] ) was reported to be 58%82% , while the specificity was 65%86%.17 the sensitivity of positive leukocyte esterase , nitrite , and leukocyte esterase plus nitrite tests was reported to be 74%96% , 35%85% , and 75%84% , respectively .
in contrast , their specificity was 94%98% , 92%100% , and 82%89% , respectively.1821 the sensitivity of positive dipstick hematuria was 44% , while its specificity was 88%.22 the sensitivity of finding of > 1 bacterium per hpf by gram stain ( centrifuged urine ) was 93% , while the specificity was 95%.23 we demonstrated that the sensitivity of pfs in diagnosing acute pyelonephritis was 72% , while the specificity was 60% .
this result shows that radiological imaging is not a particularly useful tool for the diagnosis of acute pyelonephritis .
the emergency physician , in wanting to avoid missing life - threatening diseases , may order radiological imaging , including ct scan ; this may thus contribute to an increased number of ct scans performed .
indeed , japan is reported to have one of the highest numbers of ct machines in the world.24,25 however , studies have shown that excessive use of diagnostic imaging is not necessarily associated with improved medical care.2628 in terms of cost , an abdominal ct scan ( ~\10,000 ) is much more expensive than a urine dipstick test ( \260 ) . as shown earlier ,
we believe that our findings provide a fair account of the usefulness of ct scan in diagnosing acute pyelonephritis .
the ratio of the pyelonephritis group was 0.72 and that of the control group was 0.40 ; the alpha error was 0.05 , and even if the sample size was 89 , the detection power was as high as 0.99 .
table 5 lists the various bacteria that were isolated from the positive urine cultures in our cohort ; the causative organisms were similar to previous reports in japan.29 we agree that our method of selection also created several limitations .
primary renal disease such as glomerulonephritis , tubulointerstitial nephritis , and others may increase pfs , although the cause of renal dysfunction may have little effect on the frequency of pfs ( data not shown ) .
another limitation that we did not consider was previous history of pyelonephritis , which we hope will be addressed in future studies .
another limitation of our study was the exclusion of children aged < 16 years and pregnant women .
however , this is because ct examination is not advisable for pregnant women and children .
admittedly , the occurrence of pyelonephritis is particularly high among pregnant women , with an estimated incidence of 0.5%2%.30 therefore , it was unfortunate that we could not evaluate pfs in this population of patients .
older age , male sex , and lower egfr are associated with an increased occurrence of pfs . however , with respect to the diagnosis of acute pyelonephritis , the presence of pfs itself did not have particularly high sensitivity and/or specificity compared to other tests .
 purposepyelonephritis , an upper urinary tract infection , is a serious infection that often requires hospitalization . 
 however , the accurate diagnosis of acute pyelonephritis can be difficult , especially among older individuals who can present with unusual symptoms . 
 imaging with computed tomography ( ct ) is not unusual in the diagnosis of pyelonephritis , with some clinicians regarding perirenal fat stranding ( pfs ) as a characteristic finding . 
 however , the sensitivity and specificity of pfs in diagnosing pyelonephritis are currently unknown . 
 we therefore sought to clarify the relevance of pfs in diagnosing acute pyelonephritis.patients and methodswe conducted a case - controlled retrospective analysis of medical records . 
 the pyelonephritis group included 89 patients who had been diagnosed with acute pyelonephritis , while the control group included 319 patients who had undergone percutaneous renal biopsy . 
 ct findings were available for both groups . 
 the frequency of pfs and its sensitivity and specificity for the diagnosis of acute pyelonephritis were investigated.resultsthe mean ages of the pyelonephritis and control groups were 7415 years and 6316 years , respectively . 
 a total of 28% of men were in the pyelonephritis group vs 61% of men in the control group . 
 the frequency of pfs was 72% in the pyelonephritis group vs 39% in the control group . 
 age and renal dysfunction were associated with an increased frequency of pfs . after adjusting for age , sex , and renal function using a propensity score analysis , the sensitivity , specificity , and positive likelihood ratio of pfs for diagnosing acute pyelonephritis were 72% , 58% , and 1.7 , respectively.conclusionthe presence of pfs was not useful in diagnosing acute pyelonephritis . 
Plain language summary
Introduction
Patients and methods
Study design
Data selection
Statistical analysis
Results
Discussion
Conclusion