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Memo : COMPLETED DIAGNOSIS WORKSHEET: page 1 of 2 COMPLETED DIAGNOSIS WORKSHEET: page 1 of 2 Clinical Question: In babies affected by Down's Syndrome, how useful are clinical examination, CXR, ECG in predicting the risk of congenital heart disease? Are the results of this diagnostic study valid? Was there an independent, blind comparison with a reference ("gold") standard of diagnosis? Yes. All examined by cardiac ECHO. Person doing ECHO was probably not blind to other examination results. Was the diagnostic test evaluated in an appropriate spectrum of patients (like those in whom it would be used in practice)? Yes. All babies with Down's syndrome in the region. Unlikely that any cases missed. Was the reference standard applied regardless of the diagnostic test result? Yes, eventually, most during early infancy. Are the valid results of this diagnostic study important? YOUR CALCULATIONS: Hear t Disease (ECHO) Totals Present Absent Diagnostic Test Result Positive 18 b 3 a+b 21(clinical examination) Negative 16 c d 44 c+d 60 Totals 34 47 a+b+c+d 81Sensitivity = a/(a+c) = 52.9% Specificity = d/(b+d) = 93.6%Likelihood Ratio for a positive test result = LR+=sens/(1-spec)= 8.3Likelihood Ratio for a negative test result=LR-=(1-sens)/spec= 0.5Positive Predictive Value = a/(a+b) = 86% Negative Predictive Value = d/(c+d) = 73%Pre-test Probability (prevalence) = (a+c)/(a+b+c+d) = 42%Pre-test-odds = prevalence/(1-prevalence) = 0.72Post-test odds for negative test result = Pre-test odds x Likelihood Ratio = 0.72 x 0.5 = 0.36 Post-test Probability for negative test result = Post-test odds/(Post-test odds + 1) = 26% COMPLETED DIAGNOSIS WORKSHEET: page 2 of 2 Can you apply this valid, important evidence about a diagnostic test in caring for your patient?Is the diagnostic test available, affordable, accurate, and precise in your setting? Yes Can you generate a clinically sensible estimate of your patient's pre-test probability (from practice data, from personal experience, from the report itself, or from clinical speculation) Yes. Unlikely to differ from that used in the study. Will the resulting post-test probabilities affect your management and help your patient? (Could it move you across a test-treatment threshold?; Would your patient be a willing partner in carrying it out?) Depends on results. Negative tests don't move patient across management threshold (ie. would still want ECHO). If use 26% (after exam) as new pre-test probability, post-test probability after normal CXR will be 17% (LR for -ve CXR = 0.57). 17% chance of congenital heart disease still highWould the consequences of the test help your patient? Yes. A positive examination is helpful. A negative examination still requires ECHO. Very little additional information provided by Chest X-ray or ECG if examination is negative. Additional Notes:1. It is important that the table (2 x 2) is the correct orientation, (ie. gold standard on top). Note that Table 1 on pg 1426 needs to be reoriented. 2. ECG : is a SpPin (Specificity = 100% and positive ECG rules in diagnosis) 3. Can use post-test probability of one test as pre-test probability of the next test (as long as the tests are independent). It is unlikely the clinical examination and CXR are truly independent, which overestimates the LR. 4. Emphasise the importance of thinking about the confidence intervals around likelihood ratios (CATmaker works these out for you). A LIKELIHOOD RATIO NOMOGRAM Anchor a straight-edge along the left edge of the nomogram at your patient's Pre-test Probability and pivot it until it intersects the Likelihood Ratio for your patent's diagnostic test result. It will intersect the right edge of the nomogram at your patient's Post-test Probability. Test 1: for a Likelihood Ratio of 1, Pre-test and Post-Test Probabilities should be identical. Test 2: for a Pre-test Probability of 30% and a likelihood ratio of 5, the Post-Test Probability is just under 70%. Adapted from Fagan TJ (1975) Nomogram for Bayes' theorem.N Engl J Med. 293: 257. Additional Tutor Notes : Congenital Heart Disease in Down's Syndrome Scenario : Can now work out likelihood ratios for clinical examination. Knowing prevalence, can work out post-test probability (after both tests) -see below. There is still a 17% risk of congenital heart disease (CHD) in a baby with Down's despite normal exam and CXR. However, paper states that pulmonary hypertension doesn't develop before 6 months of age, so parents can go home now. They need to know what clinical signs of CHD to look out for, and that even if their baby has symptoms or not, s/he will require an ECHO before the age of 6 months. 1. Working out the Likelihood ratiossee Table 1 (pg 1426) but note that the table is round the wrong way (need to put gold standard, ie. ECHO, on top. By creating 4 2x2 tables can generate information in Table 2. ECHO +ve -ve +ve 18 3 21 Sensitivity = 18/34 = 0.53 = 53% Clinicalexam -ve 16 44 60 Specificity = 44/47 =0.94 = 94% 34 47 81 PPV = 18/21 = 0.86 = 86% NPV = 44/60 = 0.73 = 73% Emphasize that positive and negative predictive values (PPV and NPV) change with changes in prevalence, therefore likelihood ratios are better to calculate. LR + (likelihood ratio for positive test) = Sens / (1-Spec) = 0.53 / (1- 0.94) = 8.8 or approx 9 LR- (likelihood ratio for negative test) = (1-Sens)/ Spec = (1-0.53)/ 0.94 = 0.5 ECHO +ve -ve +ve 15 1 16 Sensitivity = 15/34= 0.44 = 44% CXR -ve 19 46 65 Specificity = 46/47=0.98=98% 34 47 81 PPV = 15/16 = 0.94 = 94% NPV = 46/65 = 0.71 = 71% CXR: LR + (likelihood ratio for positive test) = Sens / (1-Spec) = 0.44/(1-0.98) = 22 LR- (likelihood ratio for negative test) = (1-Sens)/ Spec = (1-0.44) / 0.98 = 0.57 ECHO +ve -ve +ve 13 0 13 Sensitivity = 13/32 = 0.41=41% ECG -ve 19 47 66 Specificity = 47/47=1.0 =100% 32 47 79 PPV = 13/13 = 1.0 = 100% NPV = 47/66 = 0.71 = 71% LR + (likelihood ratio for positive test) = Sens / (1-Spec) = 0.41/(1-1) = infinity This is a SpPin LR- (likelihood ratio for negative test) = (1-Sens)/ Spec = (1-0.41) / 1 = 0.59 ECHO +ve -ve +ve 24 4 28 Sensitivity = 24/34=0.71 = 71% Threecombined-ve 10 43 53 Specificity = 43/47 = 0.91=91% (1/3 positive) 34 47 81 PPV = 24/28 = 0.86 =86% NPV = 43/53 = 0.81 =81% 2. This baby had negative clinical examination (LR 0.5) and negative CXR (LR 0.57) Prevalence of CHD in Down's is 34 / 81 = 0.42 = 42% Odds = probability / (1-probability) = 0.42 / 0.58 = 0.72 so, after clinical examination (LR- = 0.5), Post-test odds = pre-test odd x LR = 0.72 x 0.5 = 0.36 Probability = Odds / (Odds +1) = 0.36/1.36 = 0.26 or 26% Now, this post-test probability becomes new pre-test probability for CXR so, after CXR (LR- = 0.57), Post-test odds = pre-test odd x LR = 0.36 x 0.57 = 0.21 Odds = 0.21 implies Probability = 0.21/1.21 = 0.17 or 17% so the risk of CHD in this baby has been reduced from 42% to 17% after 2 normal examinations. Positive examinations would have greatly increased the risk.