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Investigating flank pain: Can we stay low?

Three Part Question

In [patients presenting with acute flank pain to the emergency department] can a [low dose CT scan] reliably diagnose [urinary tract stone disease]?

Clinical Scenario

A 35 year old male patient attends the emergency department with acute onset pain in left flank that is constant and radiating anteriorly. You speak to the radiologist to request a non-contrast CT scan to identify the cause of his pain, but the request is declined on grounds that the radiation dose is high and not justified and other imaging is advised. You wonder whether the dose reduction is possible and propose a bet to methodically examine the literature.

Search Strategy

Medline and Embase using NHS Evidence interface week 12th December 2011

[(Exp RENAL COLIC/) OR (renal AND colic)ti.ab OR (ureter* and colic)ti.ab OR (exp URETERAL CALCULI/) OR (urinary and calcul*)ti.ab OR (exp URINARY CALCULI/) OR (kidney AND calculi)ti.ab OR (exp KIDNEY CALCULI/) OR (flank AND pain)ti.ab OR (exp FLANK PAIN/)] AND [(computed AND tomography)ti.ab OR (exp TOMOGRAPHY, X-RAY COMPUTED/) OR (exp TOMOGRAPHY, SPIRAL COMPUTED/) OR (ct AND scan)ti.ab]
AND [(low AND dose)ti.ab OR (exp RADIATION DOSAGE/)]

Search Outcome

280 articles were identified. 22 were deemed directly relevant and their abstracts were reviewed. 7 articles were selected for the final critical appraisal. One meta-analysis and two prospective comparative studies provided highest level of evidence. All seven articles are summarised below.

Relevant Paper(s)

Author, date and country Patient group Study type (level of evidence) Outcomes Key results Study Weaknesses
Niemann et al
1001 patients in totalMeta analysis of 6 studiesSensitivity, Specificity, Accuracy 0.97 (0.95-0.98), 0.95 (0.92-0.97), 98.95%One article later removed from the analysis
Kim et al
M 79 F 42 Age 19-86 Mean 44 Prevalence Urolithiasis 87.9% Other diagnoses 7.4%Prospective comparativeSensitivity, Specificity, Sensitivity (for stone <2mm), Positive Predictive Value, Negative Predictive Value93-95%, 86%, 68-79%, 98-99%, 63-71%Spectrum Bias
Poletti et al
M 87 F 38 Age 19-80 years, mean 45 years BMI <18.5=9%, 18.5-24.9= 27%, 25-29.9= 10%, >30=10% Prevalence Urolithiasis 80.8% Other diagnoses 4.8% Prospective comparative Sensitivity, Specificity, Positive Predictive Value, Negative Predictive Value97%, 96%, 99%, 88% Spectrum Bias
Hamm et al
M 76 F 33 Age 20-84 years, Mean 49 years Prevalence Urolithiasis 73% Other diagnoses 13.7%Prospective comparativeSensitivity, Specificity, Positive Predictive Value, Negative Predictive Value 96%, 97%, 99%, 90%Spectrum Bias, unclear selection criteria, delay between index and reference test, partial verification
Mulkens et al
LDCT Group M 97 F 53, Age 18-87 years Mean 50.23 BMI 24.87 SDCT Group M 91 F 59, Age 22-90 years Mean 52.5 BMI 26.71 Prevalence Urolithiasis 52.7% Other diagnoses 15-16% Prospective quasi randomised consecutive, 87% from EDLDCT Sensitivity, Specificity, Positive Predictive Value , Negative Predictive Value: SDCT Sensitivity, Specificity, Positive Predictive Value , Negative Predictive Value 96.0-98.6%, 90.2-93.5% ,90.6- 93.5%,95.8- 98.6%: 93.7-98.8%, 94.2-98.4%, 93.7-96.4%, 94.2- 98.4% Unclear selection criteria, inappropriate reference test, delay in index and reference test, partial verification
Kluner et al
M 74 F 68 Age 18-83 years Mean 47 Prevalence Urolithiasis 72% Other diagnoses 14.8%Prospective comparativeSensitivity , Specificity, Positive Predictive Value, Negative Predictive Value 97% (92-99%), 95% (83-99%), 98%, 93%Spectrum Bias, Inappropriate reference test, Unclear selection criteria, delay in index and reference test, partial verification 59% lost to follow up
Tack et al
M 53 F 53 Age 15- 84 years Mean 45 Mean BMI 26.2 Prevalence Urolithiasis 36% Other diagnoses 12% Sensitivity, Specificity, Positive Predictive Value, Negative Predictive Value 90-95%, 90-100%, 94-100%, 93-98%Unclear selection criteria, inappropriate reference test, delay in index and reference test, partial verification


Urinary stone disease is one of the more common causes of flank pain. Studies that have reported significant alternative diagnoses are based on the use of unenhanced CT, considered as the gold standard for investigation. Even though it is a better investigation, there are concerns regarding the level of radiation exposure with a CT scan. Some patients may need multiple scans and may receive a substantial dose of radiation. Katz et al reported a mean effective radiation dose of 8.5 mSv for multi-detector CT. Significantly, 176 patients needed multiple scans reaching the maximum dose of 154 mSv. The lifetime attributable risk of developing cancer is generally 1/10 000 for every 10 mSv of radiation exposure, average across all ages and gender. However, for patients <30 years of age lifetime attributable risk is 10/10 000.Therefore it is essential to reduce the dose of radiation exposure. But that should not compromise with diagnostic quality. Over the last decade, the dosage of radiation for CT scans has gradually reduced and some of the CT examinations can now be done with a low dose, typically less than 3 mSv per examination. Liu et al were first to report the use of low dose CT (LDCT) in 2000, however their calculation of the radiation dose was incorrect and the paper was later retracted. Niemann et al later excluded this paper from their meta-analysis. There were a total of 1001 patients. Pooled sensitivity and specificity were 0.965 (95% CI 0.949 to 0.978) and 0.949 (95% CI 0.918 to 0.970), respectively. The accuracy of the test reflected by the area under the summary receiver operating curve was 98.95% (SE 0.0032). Kim et al and Poletti et al have directly compared the LDCT with standard dose CT. Both concluded that the investigation has high sensitivity and specificity for diagnosing urolithiasis when the stone size is 2 mm or 3 mm. Given that stones <5 mm have a 68% (95% CI 46% to 85%) chance of spontaneous passage, most clinically significant stones can be diagnosed with this technique. Poletti et al reported that the sensitivity falls to 50% for patients with a BMI >30 kg/m2. This finding has conflicting evidence. While Mulkens et al did not find any difference in diagnosing urolithiasis in obese and overweight patients, Hamm et al recommended 31 kg/m2 and Tack et al recommended 35 kg/m2 as the upper limit for doing an LDCT. However the number of obese patients in these studies is very small and therefore these are underpowered to establish any statistically significant result. Barring the three (level 1) studies, the rest have methodological flaws. The main reason is the use of a composite or a weaker reference standard. It will be ethically difficult to justify a study comparing standard dose CT with LDCT as this will mean excessive radiation and therefore a composite standard is used as reference test. However, some of the components of the composite reference standard, like the presence or absence of microscopic haematuria, plain abdominal film or ultrasound scan are of questionable value in diagnosing urolithiasis. Different investigation modalities were applied to different patients, which may have depended on the results of the initial CT scan, making it highly likely that the index test (LDCT) was part of patient workup. The meta-analysis of six studies by Niemann et al reported high sensitivity and specificity with LR+ of 18.9 and LR− of 0.04 for the diagnosis of urolithiasis by LDCT.

Clinical Bottom Line

Unenhanced low dose CT scan can be used for as a first line investigation for the diagnostic workup of patients with suspected renal colic.


  1. Niemann T, Kollmann T, Bongartz G. Diagnostic performance of low-dose CT for the detection of urolithiasis: a meta-analysis. AJR American journal of roentgenology. 2008;191(2):396-401
  2. Kim BS, Hwang IK, Choi YW et al. Low-Dose and Standard-Dose Unenhanced Helical Computed Tomography for the Assessment of Acute Renal Colic: Prospective Comparative Study Acta Radiologica. 2005;46(7):756-63
  3. Poletti PA, Platon A, Rutschmann OT et al. Low-dose versus standard-dose CT protocol in patients with clinically suspected renal colic. AJR American journal of roentgenology 2007;188(4):927-33.
  4. Hamm M, Knopfle E, Wartenberg S et al. Low dose unenhanced helical computerized tomography for the evaluation of acute flank pain. J Urol. 2002;167(4):1687-91.
  5. Mulkens TH, Daineffe S, De Wijngaert R et al. Urinary stone disease: comparison of standard-dose and low-dose with 4D MDCT tube current modulation. AJR American journal of roentgenology. 2007;188(2):553-62.
  6. Kluner C, Hein PA, Gralla O et al. Does ultra-low-dose CT with a radiation dose equivalent to that of KUB suffice to detect renal and ureteral calculi? J Comput Assist Tomogr. 2006;30(1):44-50.
  7. Tack D, Sourtzis S, Delpierre I et al. Low-dose unenhanced multidetector CT of patients with suspected renal colic. AJR American journal of roentgenology. 2003;180(2):305-11