Analytical Performance Flashcards
What does analytical mean?
Assess how well an instrument/method measures an analyte of interest.
What does diagnostic mean?
This is how well a given test discriminates between an individual that has a disease and a person that does not (follows analytical)
How can analytical and diagnostic performance be assessed?
PASS
Why is monitoring analytical performance important?
- quality control
- stability of performance of lab tests
- for reg bodies
- validation of methods/assays and instruments
- calibration
- comparison between labs and different instruments.
What is precision? two Rs
Repeatability and reproducibility.
What are the 3 ways in which precision can be evaluated?
Intra assay (during), inter assay (between) and precision profile. Multiple runs needed.
How can precision be numerically defined?
SD(dispersion) and CV
What does SD mean?
This is defined by the variation or the dispersion from the mean or expected value.
What are the SD bell curve graph %s?
68.3% - 1SD
95% - 2SD
99.7% - 3SD
What is more precise 1SD or 3SD?
1 SD - less of a degree of variability between measurements
What is the average adult blood glucose level?
5.5 +/- 2.2 mmol/Lt
How is SD used in clinical measurements?
- reference ranges for test analytes.
- assessment of analytical and diagnostic performance.
= depends on the clinical significance of the results generated.
How is CV calculated?
%Cv = (SD/mean) x100/1
What does CV do?
It scales the SD by the size of the mean.
Makes it possible to compare across variables measured on different scales and allows for comparison of variability
What is a with in run precision (intra assay)?
Closeness of agreement between results of successive measurements collected under identical conditions.
What is a between run precision assay?
QC material representing low, med, high conc values are assayed in groups of 20 in different analytical runs over DIFFERENT DAYS
What does a precision profile involve?
The examination of the variation of a method over a number of analyte concentrations, days and optionally over one or two runs per day.
What is the plot of precision profile?
%Cv vs conc
What can precision profiles be used to establish ?
Working reference ranges.
Ensures that imprecision is below a pre-established level.
What are the 2 requirements needed in order to construct a precision profile?
- 3 replicates must be observed for each run, and each run must have the same number of replicates.
- Known conc of analyte
What is Accuracy?
This is the nearness of a result to the true value.
Found measuring the % inaccuracy.
How can % inaccuracy calculated?
(Nominal value - experimental value)/ nominal value x100
What does matrix spiking, recovery study consist of?
Tests the ability of an assay to recover - measure a known amount of analyte from a sample matrix.
How is the concentration recovered calculated?
[c] = A - B
What is the general acceptable recovery ?
> 90%
What does accuracy depend on ?
Specificity.
What does specificity measure?
Proportion of negatives which are correctly identified (true neg)
The proportion of patients who do not have the disease who will test negative for the mutation.
The lower the specificity the higher the ?
Chance of a false positive.
How is specificity calculated?
true neg/ true neg + false pos. NEED to BE HIGH
What is sensitivity ?
This is the ability to identify a positive results correctly.
The higher the sensitivity the lower the ?
Chance of a false negative.
What is the Limit of Detection Limit?
This is the the lowest quantity of a substance that can be distinguished from the absence of that substance with a degree of confidence.
What can a sensitive assay do?
Measure low levels of analyte.
How can sensitivity be calculated?
Opp of specificity - true pos used.
What are the 3 situations in which clinical labs need to compare methods?
- Purchase of a new analyser
- New method introduced.
- A quicker, more convenient or more economical adaptation to an existing interpretation.
What are the 2 kinds of data that can be used to compare methods ?
- independent data about PASS
- measurements that are made by both methods on the same clinical samples.
What are the 3 factors required for comparing methods?
- sufficient time for familiarisation
- samples cover the clinically encountered range of results
- sufficient number of samples analysed
How are statistical tests applied?
To describe the relation between results by two methods: paired T tests, least square linear regression, Pearson correlation.
