Statistics and QC part 2

Question 0

this is the agreement among replicate measurements; can be only determined with multiple measurements; reproducibility

Answer 0

precision

Question 1

this is the agreement between the measured quantity and its true value; can be determined by only one measurement; closest to true value

Answer 1

accuracy

Question 2

precision is determined by calculating the ________ and coefficient of variation statistics.

Answer 2

standard deviation

Question 3

what are three guidelines used to establish limits for accuracy and precision in the clinical lab?

Answer 3

desirable limits for accuracy and precision must be defined 1 individually for each test or type of analysis performed 2 at each level of medical significance and 3 with regard to the flow of work in the lab

Question 4

the difference between the measured value and the true value and assesses uncertainty in measurement

Answer 4

error

Question 5

the amount of analytical imprecision and systematic error permitted for a single test result; also a point at which the results from the assay may no longer by useful

Answer 5

total error

Question 6

unpredictable error including precision, imprecision, reproducibility, and repeatability

Answer 6

random error

Question 7

what can cause random error?

Answer 7

1 instability of the instrument
2 variations in the temp
3 variations in the reagents and calibrators
4 variability in handling techniques, such as pipetting, mixing, and timing
5 variability in operators

Question 8

this describes error that is consistently high or low and is predictable; caused by factors like deteriorating reagents or an instrument that has lost calibration; introduce bias

Answer 8

systematic error

Question 9

What are causes of constant systematic error?

Answer 9

1 presence of interfering substances in all samples or reagents
2 presence of interfering substances in the reaction between the analyte and reagent
3 presence of interfering substances that destroy or inhibit the reagent

Question 10

What are causes of proportional systematic error?

Answer 10

1 SD changes from reagent lot to reagent lot

2 SD changes from calibration to calibration

Question 11

What is the calculation for SDI? What is it used for?

Answer 11

(Lab mean-Group Mean)/Group SD = SDI

USed to determine systematic error or bias

Question 12

A target SDI is _____. An SDI of _____ or less is considered acceptable. SDI of _____ or greater is generally considered to be unacceptable performance and remedial action is required.

Answer 12

0.0; 1.25; 2.0

Question 13

This quality control is used in real (actual) time to ensure day to day consistency of measurements.

Answer 13

Internal QC

Question 14

This QC refers to a process of retrospectively comparing results from different laboratories through the use of an external agency; major purpose is to assess accuracy by comparing results with other labs with similar methods.

Answer 14

External QC

Question 15

What is the target range?

Answer 15

95.5% confidence limits of the data or +- 2 SD

Question 16

What are the # of data points, # of runs, and # of days required by NCCLS for a new lot of QC?

Answer 16

at least 20 data points from 20 separate analytical runs over no more than 30 days

Question 17

What are the # of data points, # of runs, and # of days required by NCCLS to establish a target range for a new QC system or manufacturer?

Answer 17

40 data points from at least 20 analytical runs collected over no more than 30 days

Question 18

this is an abrupt change in the mean that becomes continuous; it is evidence of systematic error and must continue for 5-7 data points

Answer 18

shift

Question 19

What can cause a shift?

Answer 19

1 sudden failure or change in the light source
2 change in reagent formulation
3 change of reagent lot
4 major instrument maintenance
5 sudden change in incubation temperature
6 change in room temp or humidity
7 failure in the sampling or reagent dispense system
8 inaccurate calibration/recalibration

Question 20

this is a gradual change in the mean that proceeds in one direction; it is an example of systematic error; continues for 5-7 data points

Answer 20

trend

Question 21

What are possible causes for trends?

Answer 21

1 deterioration of the instrument light source
2 gradual accumulation of debris in sample/reagent tubing
3 gradual accumulation of debris on electrode surfaces
4 aging of reagents
5 gradual deterioration of control materials
6 gradual deterioration of incubation chamber temp
7 gradual deterioration of light filter integrity
8 gradual deterioration of calibration

Question 22

What is the calculation for percent recovery? and what is an acceptable range?

Answer 22

Experimental Result/Theoretical Result x 100 with a range of 95%-105% being acceptable

Question 23

Proportion of individuals with that disease who test positively with that test

Answer 23

Diagnostic sensitivity

Question 24

Proportion of individuals with out the disease who test negatively for the disease

Answer 24

Diagnostic specificity

Question 25

Number of diseases individuals with a positive test

Answer 25

True positive

Question 26

Number of individuals without the disease with a negative test

Answer 26

True negative

Question 27

Predictive value of a positive test (PV): fraction of positive tests that are true positives…formula?

Answer 27

TP/ (TP+FP) x100

Question 28

Predictive value of a negative test: fraction of neg tests that are true negatives…formula?

Answer 28

TN/ (FN+TP) x100

Question 29

Efficiency of a test is the fraction of all test results that are either true positives or true negatives…formula

Answer 29

(TP+TN) / (TP + TN + FP + FN) x100