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Clinical Decision on a Diagnostic Test

Inna Mangalindan. Block N. Class 2009.

September 15, 2008

+Salient Features

R.E.

25/F

Presenting with R flank pain radiating to the R inguinal area, hematuria, and frequency

+Differential Diagnosis

Urolithiasis (pretest probability of 60%) R flank pain radiating to the R inguinal area Hematuria Frequency

Acute Uncomplicated Cystitis (pretest probability of 30%) Hematuria Frequency

Acute Appendicitis (pretest probability of 2%) R flank pain (a remote possibility)

+Review

Decision threshold Based on your own clinical judgment

Pretest Probability Based on prevalence studies

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UROLITHIASIS Decision Threshold: 5%-75%

5% (lower limit) – below which, send pt home and do nothing

75% (upper limit) – start treatment Pretest Probability: 60%

This falls in between 5 and 75% so we cannot start treatment right away nor send the patient home. Therefore, we need further diagnostic testing.

0% 5% 75%

60% Urolithiasis

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ACUTE UNCOMPLICATED CYSTITIS Decision Threshold: 5%-40%

5% (lower limit) – below which, send pt home and do nothing

40% (upper limit) – start treatment (Ofloxacin 200mg BID x 3 days)

Pretest Probability: 30% This falls in between 5 and 40% so we cannot start

treatment right away nor send the patient home. Therefore, we need further diagnostic testing.

0% 5% 40%

30% Acute Uncomplicated Cystitis

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ACUTE APPENDICITIS Decision Threshold: 5%-90%

5% (lower limit) – below which, send pt home and do nothing

90% (upper limit) – start treatment (appendectomy) Pretest Probability: 2%

There is no need for further diagnostic testing in this case because it is below our 5% limit.

0% 5% 90%

2% Acute Appendicitis

+What Influences our Decision Thresholds: A Review

Physician expertise

Health system

Intervention

Prior experience

Patient preference

+Diagnostic Options for Urolithiasis

KUB (Kidney, Ureter, Bladder) Ultrasound

Intravenous Pyelography

+Diagnostic Dilemma

What is the sensitivity and specificity of KUB Ultrasound vs Intravenous Pyelography in diagnosing Urolithiasis using Cross-Sectional Studies?

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CRITICAL APPRAISALIs the study valid?

+PRIMARY VALIDITY GUIDES

1. Was there an independent and blind comparison with a reference standard?

YES. All of the 108 patients enrolled in the study first underwent KUB-UTZ then Intravenous Pyelography (the criterion standard).

A reference standard for a diagnostic test is the test that gives the information nearest to the “truth”. Thus the accuracy of the test should be compared against the standard.

+PRIMARY VALIDITY GUIDES

2. Did the patient sample include the spectrum of patients to whom the test will be applied in practice?

YES. The patients enrolled in the study were adult patients between the ages of 18 and 65 years presenting with unilateral flank pain and hematuria who presented at the Emergency Department.

The patient in our case was a 25-year-old female, presenting with R flank pain and hematuria, who went for consult at AMBU.

The accuracy of a diagnostic test among patients with low risk for the disease is different from patients with high risk of the disease.

+SECONDARY VALIDITY GUIDES

3. Was the reference standard done regardless of the result of the diagnostic test being evaluated?

YES. All 108 patients included in the study underwent both KUB-UTZ and Intravenous Pyelography regardless of the result of each test.

VERIFICATION BIAS: In some studies, the reference standard (in this case, IVP) is only done based on the initial result of the diagnostic test (KUB-UTZ) in order to verify the initial finding, i.e., when positive. When this happens, most of the data available will be those positive for the diagnostic test and the reference standard. This will increase the accuracy of the test.

+SECONDARY VALIDITY GUIDES

4. Were the methods for performing the test described in sufficient detail to permit replication?

YES. The procedures (KUB-Ultrasound and IVP) were described in detail in the methods section.

+OVERALL, IS THE STUDY VALID?

YES. Most of the questions were answered adequately.

+CRITICAL APPRAISAL

What are the results?

+RESULTS

1. What were the likelihood ratios for the different possible test results?

Likelihood ratios indicate by how much a given test result increases the pre-test probability of the disease.

5%0% 75%

60% Urolithiasis

+Likelihood Ratios for the Diagnostic Test (KUB-UTZ)

Positive Test

LR (+) = Sn/1-Sp 0.971 / (1 – 0.589) 0.971 / 0.411 2.363

Negative Test

LR (-) = 1-Sn/Sp (1 – 0.971) / 0.589 0.029 / 0.589 0.049

+Nomogram for interpreting diagnostic test results (Likelihood ratio)

In this nomogram, a straight line drawn from a patient's pre-test probability of disease (which is estimated from experience, local data or published literature) through the LR for the test result that may be used, will point to the post-test probability of disease.

+Post-test Probabilities after KUB-UTZ

82% now falls on the decision to treat limit. Therefore, a positive KUB-UTZ result can boost your decision to start treatment already.

However, a negative KUB-UTZ result is not enough to make you decide to send your patient home because it is still above the lower limit of our decision threshold.

60% Urolithiasis

0% 5% 75%

7% 82%

(-) (+)

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CRITICAL APPRAISALWill the results help me in caring for my patients?

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Will the reproducibility of the test result and its interpretation be satisfactory in my setting?

YES. KUB-UTZ is widely used in our setting. It was also adequately described in the study and its interpretation was definite and simple.

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Are the results applicable to my patient?

YES. Patients included in the study are those between the ages of 18 and 65 years who presented at the Emergency Department with unilateral flank pain and hematuria.

Our patient is a 25-year-old female who presented at AMBU with R flank pain, hematuria, and frequency.

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Will the results change my management?

YES. If the KUB-UTZ is positive, the post-test probability increases to 82%, which falls into the range for a decision to start treatment.

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Will patients be better off as a result of the test?

YES. Doing the test will help us in deciding whether to start treatment.

+Resolution of the Problem in the Scenario

KUB-UTZ has a high sensitivity and can be used as an effective screening tool in the evaluation of adult patients presenting with unilateral flank pain and hematuria.