Analytical Performance Flashcards
define analytical and diagnostic performance
Analytical: assesses how well an instrument/method measures analyte of interest
Diagnostic: assesses how well a test can differentiate between a healthy/unhealthy patient
what 4 things can help assess analytical and diagnostic performance
precision, accuracy, specificity, sensitivity
why is there need to monitor analytical performance in clinical labs
- ensures reliability of data (quality control) to assess stability of lab tests
- meet reqs of regulatory bodies
- validation (characteristics of assay), calibration (maintain accuracy), comparison ( compare between diff labs)
what is an inter-assay
repeating assessment on a diff day to ensure precision
what is a precision profile
in depth analysis of precision over a period
what is SD
a numerical value that indicates the extent of deviation for a defined data set- can help define how much variation occurs from the mean
what does a low/high SD value mean
the data points are close to the mean= low
the data points are spread form the mean = high
what is the formula for SD
see notes
what is the clinical significance of SD
- can assess analytical + diagnostic performance
- ref ranges in lab are usually established as the mean +- 2SD
what is the coefficient of variation
- scales the standard deviation by size of the mean
- makes it possible to compare across variables measured on diff scales
- allows comparison regardless of magnitude of analyte conc
wjat is the formula for CV%
SD/mean x 100
what is with-in-run precision
intra assay - one analyst
obtains the closeness of agreement between results of successive measurements obtained under identical conditions
i.e samples representing low, medium and high conc values are assayed in replicates of n=20 on the same day
what is between run precision
inter assay- maye have more than one analyst
i.e QC material representing low med and high conc valyes are assayed in replicates of around n=20 in diff analytical runs over a number of days
write a note on precision profile
is an extensive assessment of precision
- examine method variation over a number of analyte concs, days and optionally over 1/2 runs per day
- make a plot of %CV v Conc
- this helps make working ref ranges
- ensure that imprecision is below establised level
what are the requirements for precision profile
at least 3 replicates must be observed for each run + each run must have the same number of replicates
analyte conc must be known
how can accuracy be quantified
measuring inaccuracy or %error
inaccuracy = nominal value - experimental value
% inaccuracy = nominal value - exp value / nominal value x 100
define accuracy and precision
accuracy = proximity of results to true value
precision = repeatability of measurement
what is recovery study (matrix spiking)
tests ability of an assay to measure a known amount of analyte from a sample matrix
to do this, add known cocn of analyte (A)to sample matrix and measure recovered conc (C)
to account for matrix background generate a baseline measurement (B) i.e measure in absence of A
conc recovered = A-B
% recovery = (C/A) x 100
define specificity
measures proportion of negatives that are correctly identified
low specificity = high chance of false pos
can be determined by measuring cross-reactivity with molecules that are chemically similar i.e antigen/antibody reactions
what is formula for specificity
number of true negs/ number of tru negs + number of false pos
what is sensitivity
relates ability to identify pos results correctly
higher the sensitivity = lower chance of false neg result
what is the limit of detetion
the lowest quantity of a substance that can be distinguished from absence of that substance with degree of confidence
i,e a sensitive assay can measure low levels of the analyte
why may method comparison be needed
-purchase of a new analyser, new method introduced, more efficient/economical adaptation to existing method
what sort of data can be used to compare methods?
data from both methods representing the same clinical samples
independent data- diff clinical samples that represent similar study populations
what is needed for method comparison
sufficient time for familiarisation, samples cover the clinically encountered range of results, sufficient number of samples analysed
