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RES 131 Reference Values

RES 131 Reference Values. Reference Values. Interpretation of the values obtained from Pulmonary Function Testing is difficult without a standard for comparison Therefore, reference or normal values have been established for comparison with individual performance. Reference Values.

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RES 131 Reference Values

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  1. RES 131Reference Values

  2. Reference Values • Interpretation of the values obtained from Pulmonary Function Testing is difficult without a standard for comparison • Therefore, reference or normal values have been established for comparison with individual performance

  3. Reference Values • Studies designed to develop reference values test large numbers of participants with “normal lungs” • Non-smokers • No present or past acute or chronic lung disease. • From the study “Regression Equations” are developed.

  4. Reference Values • If a large group of individuals was tested, who had similar characteristics, The testing values for most participants would fall near the mean value for the group. • Because of normal variations within a population, some of the participants would be higher and some lower.

  5. Reference Values • If the values were plotted on a graph, a bell shaped distribution would be created. • The width of this distribution would be described by its standard deviation.

  6. Reference Values

  7. Reference Values • In interpreting Pulmonary Function values, generally the main concern is whether a patients performance is below the lower limits of normal. • An exception is when interpreting lung volumes in the presences of obstructive disease.

  8. Reference Values • Two methods for comparison of patient performance against reference values • 95% Confidence interval • 80% method

  9. References Values • 95% confidence interval • performance values below the fifth percentile (lowest 5% of reference population) is considered below the expected range (below normal)

  10. Reference Values • Calculation of the 95% confidence interval • Requires the value of SEE (standard error of estimate) from the reference study. • Calculated as : • 1.64 x SEE = 95% CI

  11. Reference Values • Determining the lower limit of normal using the 95% CI • Predicted value (mean of the reference population value) minus the 95% CI (SEE x 1.64)

  12. Reference Values • Example: • predicted value = 4.70 • SEE = 0.74 • 4.70 - (1.64 x 0.74) = 3.49

  13. Reference Values • 80% Method • A simple and commonly used method used to establish limits of normal is the 80% method. • Lower limit of normal equals: • predicted value x .80 • Example: 4.70 x .80 = 3.79

  14. Selecting Reference Values • Two Main Factors to consider: • Population used in the reference study • Methods and instrumentation used in the reference study.

  15. ATS Statement on Predicted Values • Reference values should be bases on data obtained using ATS guidelines • Reference equations should include age and height as independent variables • Reference equations should not be extrapolated for ages or heights beyond the stated range.

  16. ATS Statement on Predicted Values • Should take into account differences in ethnic origins of study population and the patients in the laboratory. • Reference equations should have information to determine the lower limit of normal • Check the study against 20 to 40 healthy subjects in your lab.

  17. Race and Ethnic Considerations • The size of the lungs relative to body size varies with race. • May be due to: • trunk length to height differences, free fat mass, chest dimensions and maximal inspiratory and expiratory pressures.

  18. Approaches to Race and Ethnic Differences • Use race specific reference values • not always practical • Application of a correction factor • Have used a 15% reduction for all none white patients in the past • Not a valid method due to vast differences • Current ATS recommendation is a 12% reduction for black patients • Comments in the interpretation

  19. Following Changes Over Time • It is also important to compare patient test results over time. • Comparing changes over time is often more valuable than comparing to reference values

  20. Following Changes Over Time

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