ECHONOMY
Tools for Echocardiographic Calculations
Muhamed Saric, MD, PhD
New York University


Purpose

To
demonstrate the impact of disease prevalence (pretest probability) on
positive and negative predictive values given the sensitivity and the
specificity of the test. Although the calculator is used in the context of
stress testing for coronary artery disease, basic principles apply to any
testing. 
Major Points

No matter how good an interpreting
physician is, some stress test results will be false positive and some false
negative. The primary determinant of the number of falsely read stress tests
is the prevalence of the coronary artery disease (CAD) in the test
population. 
Calculator

Disease prevalence
(pretest probability of disease) 

% 
Low: <10%
Moderate: 20  30%
High: >30% 




Sensitivity 

% 
XSE = 74  97% (mean 88%)
DSE = 61  95% (mean 81%)
PSE = 61  81% (mean 77%) 




Specificity 

% 
XSE = 64  86% (mean 79%)
DSE = 51  95% (mean 80%)
PSE = 90  94% (mean 92%) 
XSE, exercise stress echo
DSE, dobutamine stress echo
PSE, persantine stress echo 



Answers

Positive predictive value (PPV) 

% 
Negative predictive value (NPV) 

% 
False positives 

% 
False negatives 

% 



Interpretation

PPV: Proportion of all positive stress tests that are truly positive, i.e.
likelihood that a positive stress test is correct in identifying the
disease.
NPV: Proportion of all negative stress tests are truly negative; i.e.
likelihood that a negative stress test is correct in ruling out the disease.
Stress testing works best when the prevalence (pretest probability) of CAD
is moderate (10  30%). In lowrisk populations, there is a large number of
false positives and only a few false negatives. In highrisk populations,
there is a large number of false negatives and a few false positives.

Formula
Sensitivity = TP / P = TP / (TP + FN)
Specificity = TN / (1  P) = TN / (TN + FP) PPV = TP / (TP
+ FP) NPV = TN / (TN + FN)
Abbreviations: TP, true positives; FP, false positives; TN, true negatives;
FN, false negatives; P, prevalence (pretest probability). 
Reference

Kline JA, Johnson CL, Pollack CV Jr,
Diercks DB, Hollander JE, Newgard CD, Garvey JL. Pretest probability
assessment derived from attribute matching.
BMC Med
Inform Decis Mak. 2005 Aug 11;5:26.
Marwick TH.
Stress echocardiography.
Heart.
2003 Jan;89(1):1138.
