Statistical analysis Flashcards
Technical/analytical evaluations to be performed when a new test is to be introduced to the routine lab
- Estimation of assay imprecision
- Comparison of new vs old assay
- Assessment of allowable measurement range - estimation of upper and lower limits of quantification
- Evaluation of interferences and carry over if applicable
- Assess whether it meets lab accreditation requirements and/or other regulatory standards.
Define CVa
Analytical coefficient of variation
Define CVg
Between subject coefficient of variation
Define CVi
Within-subject coefficient of variation
Define CVrb
Sample-related random bias coefficient of variation
Two types of biological variation?
Within subject and between subject
What is within-subject variation?
Random fluctuations around a homeostatic point
How is biological variation determined?
Using a biological variation study
How is a biological variation study performed?
Recruit study subjects
Take serial samples at intervals consistent with the intended use of the tests
Analyse the samples in duplicate
Perform an Analysis of Variance (ANOVA)
Variations that affect the test results of an individual person over time?
- Pre-analytical variation (CVp)
- Analytical variation (CVa)
- Within subject variation (CVi)
Variations that affect the test results of an individual person over time?
- Pre-analytical variation (CVp)
- Analytical variation (CVa)
- Within subject variation (CVi)
What is pre-analytical variation?
Variation due to differences in patient preparation between tests, sample collection and conditions that the collected sample experiences after collection but prior to measurement itself
What is reference change value?
Helps us understand if the change in a patient’s serial result is statisically significant. Determined from Z-score, CVa and CVi. Assumes CVp negligible.
Formula for RCV
Formula for index of individuality
CVi/CVg
What is the index of individuality used for?
Determining if population based reference intervals are useful for a patient. If the II is <0.6, population based reference intervals are not useful (within subject variation is very low and a change within the reference interval may still be significant). If the II is >1.4 then population based reference intervals are helpful.
The index of individuality for troponin I is 0.29. What is the implication of this for interpreting troponin I results?
The low index of individuality means that within an individual, troponin levels do not show much fluctuation. The utility of population based reference intervals for determining the significance of a troponin I value is low. Instead, serial measurements will be of importance to determine if a change is significant or not.
How is total method error calculated?
From bias and standard deviation established by method comparison experiments. TME = Bias + 1.65SD
How are total allowable error goals determined
Legal criteria
Targets set by providers of proficiency testing/EQA schemes
Using biological variation data
How is CV calculated?
CV% = SD/Mean x 100%
Biological variation database provided by?
European Federation of Clinical Chemistry and Laboratory Medicine
How should desirable imprecision (CVa) be calculated?
Less than 0.5CVi
How should desirable bias be calculated?
Bias should be less than 0.25 total biological variation
How is total biological variation calculated?
Square root of (CVi^2 +CVg^2)
How is total allowable error calculated?
TME + Bias
= 1.65ximprecision goal + bias goal
= 1.65 x 0.5CVi + SqRt(CVi^2+CVg^2)
