Harvey & Stockham - Stats, QA/QC - AKG

Question 1

When determining reference intervals and a Gaussian distribution is present, a minimum of ___ individuals should be assayed for statistical validity.

Answer 1

40

Question 2

____ is the likelihood of a positive or abnormal test result occurring in animals with the disease being considered.

Answer 2

Sensitivity

*trust your negatives - negative result means you can rule the disease OUT

Question 3

_____ is the likelihood of obtaining a negative or normal test result in a nondiseased animal.

Answer 3

Specificity

*trust your positives; if positive - you can rule the disease in

Question 4

Calculate Sensitivity (%)

Answer 4

(TP x 100)/(TP + FN)

*calculates likelihood of a positive or abnormal test result occurring in animals with the disease (denomater = total diseased animals = TP + FN)

Question 5

Calculate Specificity (%)

Answer 5

TN x 100/(TN + FP)

*likelihood of obtaining a negative or normal test in nondiseased animals (denominator = total healthy animals = TN + FP)

Question 6

Please define and calculate PPV (aka PVPT).

Answer 6

Positive predictive value considers only animals in the population being studied that have a positive test result and determines what % of animals actually have the disease being considered.

PPV = TP x 100/ (TP + FP)

(denominator = all positive test results)

Question 7

Please define and calculate NPV (aka PVNT).

Answer 7

Predicted value of a negative test (NPV) consideres only animals in the population being studied that have a negative or normal test results and determines what % of animals with negative test results do not have the disease being considered.

How likely is that an animal with a negative or normal test result will be free of the disease being considered?

PVNT (NPV) = TN x 100/ (TN + FN)

Question 8

Define prevalance of a disease. Calculate using TP/FP/TN/FN

Answer 8

The percentage of animals in a given population that have a certain disease.

(TP + FN) x 100/ (TP + TN + FP + FN)

Question 9

What affects prevalance:

A) Predictive values

B) Sensitivty and specificity

Answer 9

A) Predictive values

Question 10

A low prevalance will keep the PVPT ___ and PVNT ____. The exception is tests with rare false positives, like PCR tests.

Answer 10

low and high

Question 11

Tests with high positive predictive values are needed for a rule__ strategy when significant hazards are associated with treatment or euthanasia is being considered.

Tests with a high negative predictive value are theoretically important as a rule-___ startegy when the penalty for missing a dx is high (as with a disease for which terhapy is effective if begun quickly).

Answer 11

rule-in

rule-out

Question 12

The best evidence for ruling out a disease is finding a negative test result for an assay that has a high _____ for the recognized disease.

Answer 12

sensitivity

Question 13

____ is determined by how closely the result approaches the true value of the analyte being measured.

Question 14

Low accuracy are said to have a negative or positive ____ based on if results are below or above the true value, respectively.

Answer 14

Bias

Question 15

the ____ of a test reflects how reproducible the test results are when the assay is replicated.

Answer 15

precision

Question 16

Precision is independent of accuracy. T/F

Answer 16

True

Question 17

What is the coefficient of variation (CV)?

Answer 17

the amount of imprecision (aka random error) present in an assay

More specifically, it is the standard deviation of the repeated measurements expressed as a percent of the mean of the repeated measurements (SD/mean x 100)

It can be measured over time intervals to assess within-run, between-run, or between-day variation.

Question 18

CV depends on accuracy. T/F

Answer 18

False. It measures impercision (aka random error), and therefore depends on precision.

Question 19

What do Bland Altman plots show?

Describe the y and x axis.

What do the horizontal lines mean?

Answer 19

Bland-altman plot (aka difference plot) is a graphical method to compare two measurement techniques.

The differences between the two techniques (usually y axis) are plotted against the averages of the two techniques (usually x axis). Alternatively, the differences can be plotted against one of the two methods, if this method is a reference or “gold standard” method.

Horizontal lines are drawn at the mean difference, and at the limits of agreement (SD). The line of equality is at 0 on the y axis (where measurements of method 1 = method 2).

Question 20

The graph is an example of ______ error/bias.

Answer 20

proportional

Question 21

Interpret this bland-altman plot

Answer 21

Variation of at least one method depends strongly on the magnitude of measurements.

Question 22

The graph is an example of ______ error/bias.

Answer 22

constant

Question 23

This test is a robust, nonparametric method for fitting a straight line to two-dimensional data where both variables, X and Y, are measured with error (y = bx + a). It is useful when you have two devices that should give the same measurements and you want to compare them.

Answer 23

Passing Bablok regression for method comparision

Question 24

How do you interpret Passing Bablok regression for method comparision ?

Answer 24

If 0 is within the confidence interval (CI) of the y-intercept, then there is no evidence of constant bias.

If 1 is within the CI for the slope, then you infer that there is no proportional bias.

If both are met, then the two methods are comparible within the investigated concentration range.

Question 25

EDTA chelates which ions?

Answer 25

Calcium, Mg, Cu, and Pb

Question 26

EDTA serves as an anticoagulant becuase it chelates calcium, which is a cofactor for many parts of the coagulation cascade. What is the main mechanism of action for heparin as an anticoagulant?

Answer 26

Heparin chelates calcium as well, but it’s major action as an anticoagulanat is by activating antithrombin III –> this will inhibit coagulation factors including thrombin.

Question 27

What are the three major disadvantages of using heparin for blood smear evaluation?

Answer 27

1. Alters morphologic features and staining of leukocytes
2. Allows clotting as effects are slowly overriden by the coagulation system
3. Allows platelet clumps to form

Question 28

Plasma consists ~ 95% water and ~ 5% solids. List some of the components that contribute to total solids.

Answer 28

• Mostly protein
• GLucose
• Urea
• Electrolytes

Question 29

When evaluating serum, when should you centrifuge the blood sample?

Answer 29

After clot retraction - usually takes ~30 minutes naturally, but clot activator will speed up the clotting time

Question 30

Which is higher, serum [K+] or plasma [K+]?

Answer 30

Serum [K+] - cells (especially platelets) will release intracellular contents during clotting

RIs take this into consideration, and this is only a problem if patient has thrombocytosis

Question 31

The major protein (on a weight/volume basis) that is absent in serum but present in plasma is _____.

Answer 31

Fibrinogen

Question 32

How many significant figures do the numbers below have:

1. 124
2. 124.0
3. 0.12
4. 0.020
5. 120

Answer 32

1. 124 - 3 SF
2. 124.0 - 4 SF
3. 0.12 - 2 SF
4. 0.020 - 2 SF
5. 120 - 2 or 3 SF

Question 33

When performing calculations, the answer should have the _____ (fewest or most) significant figures present in the values used in the calculation.

Answer 33

fewest

e.g. 1.23 x 2.4 = 2.952 –> report as 3.0 becuase 2.4 has the fewest significant figures at 2.

Question 34

If values have a Gaussian distribution, the mean, median, and mode values will be _____.

Answer 34

Equal

Question 35

If the reference distribution is positive skewed, the tail of the graph pulls out to the _____ and the peak of hte graph is shifted to the ____. List in order, highest from lowest value, the mean, median and mode.

Answer 35

right

Left

Mode > median > mean

Question 36

If the reference distribution is negatively skewed, the tail of the graph pulls out to the left and the peak of the graph is shifted to the right. List in order, lowest from highest, the mean, median and mode.

Answer 36

mean < median < mode

Question 37

Sample collection and sample handling are examples of _____ errors.

Answer 37

Preanalytical

Question 38

List examples of analytical error

Answer 38

• Method appropriate for species
• Quality of instruments and equipment
• Quality of reagents
• Quality of laboratory technique
• Quality control program

Question 39

Transcriptional erros and incorrect interpretation are examples of _______ errors.

Answer 39

Postanalytical

Question 40

When evaluating and comparing laboratory assays, five properties can be assessed. What are those five properties?

Answer 40

1. Analytical precision
2. Analytical accuracy
3. Analytical specificity
4. Analytical sensitivity
5. Detection limit

Question 41

Analytical ____ is the ability of an assay to get the same result if a sample is analyzed several times. Aka random analytical error or reproducibility.

Answer 41

precision

*analytical impercision is expressed by CV

Question 42

Calculate CV %

Answer 42

Standard deviation/mean x 100

Question 43

A ____-assay CV represents the random error that is expected when one sample is analyzed multiple times in one run of an assay.

A _____-assay CV represents the random error within one run of the assay plus the error from additional runs of the assay by using the same sample.

Which of the assays will be smaller?

Answer 43

within-assay

between-assay

within-assay will be the samller value

Question 44

An assay’s CV will typically vary with the analyte’s concentration. Higher CV values may be found at the lower and upper limits of the assay’s analytical range. WIthin the analytical range, CV values are typically _____ at the lower analytic concentrations becuase CV values are expressed at percentages.

Answer 44

higher

E.g., assay has a CV of 10% at all concentrations –>

• then assay’s SD would be 0.1 mg/dL at an analyte concentration of 1 mg/dL
• and 10 mg/dL at an analyte concentration of 100 mg/dL

E.g., assay has a CV of 10 mg/dL at all concentrations

• CV would be 50% at a mean concentration of 20 mg/dL
• CV would be 10% at a mean of 100 mg/dL

Question 45

_____ is an average of SD values from 3-6 consecutive months of quality assurance values.

Answer 45

USD

Question 46

If change in patient’s data is < 2 x USD –> the changes can be explained by _____ variation alone

If the change in patient’s data is > 3 x USD –> the changes are likely due to _____ variation.

Answer 46

analytical variation

biological variation

Question 47

How are control solutions used to assess precision?

Answer 47

Repeated analysis of control solutions

Question 48

Analytical _____ is defined by the closeness of the agreement between the measured value of an analyte and its ‘true’ value.

Answer 48

accuracy

Question 49

How do you test for analytical accuracy?

Answer 49

Typically assessed by comparison of the assay’s results to results of an accepted reference method, a method that has been accepted by a standardization group as providing a true value.

You can measure the analyte using a reference standard solution whose concentration was determined by a reference method

Question 50

Analytical _____ defines the ability of an assay to detect only the substance of interest (analyte) or freedom from interfering substances.

Answer 50

Specificity

Question 51

Analytical specificity is related to analytical ____.

Answer 51

accuracy

Question 52

_______ is defined by the smallest concentration or quantitiy of an analyte that can be detected with reasonable certainty for a given analytical range.

Answer 52

Detection limit

Question 53

Analytical ______ is defined by the slope of the calibration curve and the ability of an analytical procedure to produce a change in the signal for a defined change of the quanitity

Answer 53

sensitivity

in other words - how much change of the analyte is needed for the assay to detect the change

Question 54

In Westgard rules, what does 1_2s rule mean?

Answer 54

A run is rejected when a single control measurement exceeds the mean plus 2 SDs or the mean minus 2SD of previous control sample values, or it serves as a warning system to initiate additinol inspection of control data.

Question 55

In Westgard rules, what does 2_2s rule mean?

Answer 55

A run is rejected when two consecutive control measurements exceed the same mean plus 2SD or the same mean minus 2SD of the previous control sample values.

Question 56

In Westgard rules, what does 1_3S rule mean?

Answer 56

A run is rejected when a single control measurement exceeds the mean plus 3s or the mean minus 3s of previous control sample values.

Question 57

In Westgard rules, what does R_4s rule mean?

Answer 57

A run is rejected when one control measurement in a group exceeds the mean plus 2s and another exceeds the mean minus 2s

Question 58

In Westgard rules, what does 4_1s rule mean?

Answer 58

A run is rejected when four consecutive control measurmements exceed the same mean plus 1s or the same mean minus 1s control limit.

Question 59

In Westgard rules, what does 10x rule mean?

Answer 59

A run is rejected when ten consecutive control measurements fall on one side of the mean

Question 60

Which westgard rules are sensitive to systematic error vs random error?

• 1_3s
• 2_2s
• R_4s
• 4_1s
• 10x

Answer 60

• 1_3s - random error
• 2_2s - systematic error
• R_4s - random error
• 4_1s - systematic error
• 10x - systematic error

Question 61

A common method of monitoring the results of the control samples is to plot them in a ________. These are used to monitor values, but do not help us decide what is acceptable or unacceptable variation.

Answer 61

Levey-Jennings control chart

Question 62

Sigma stands for the greek letter σ - it stands for _____ and is a statistical index used to express random deviation from the mean.

Answer 62

standard deviation

Question 63

A sigma classifcation for a test refers to its _____ relative to the amount of acceptable error. The higher the sigma value, the ____ (more or less) precise and ____ (more or less) random error there is.

Answer 63

A sigma classifcation for a test refers to its precision relative to the amount of acceptable error. The higher the sigma value, the more precise and less random error there is.

Question 64

For a theoretical assay B that has a TEa of 10 mg/dL, analysis of control solution with an analyte concentration near the decision threshold reveals a SD of 2 mg/dL. Assuming there is no systematic error (bias), this assay woudl be called a ______-sigma assay. In this system, the ideal assay is is a ____-sigma assay because unacceptable error would occur only 0.002 times per million assays.

Answer 64

five

six

Question 65

What does TEa stand for?

Answer 65

allowable total error - calculated based on biological variatoins and the decision threshold

Question 66

What is the difference between NCCLS Bias plot and the bland-altman bias plot?

Answer 66

The bland-altman plot plots the mean of methods 1 and 2 on the x axis, while the NCCLS Bias plot plots values for one method on the x axis. The NCCLS bias plot is preferend when the second method is being compared to a reference method or an established method.

Question 67

The NCCLS bias plot or the bland-altman bias plot displays a positive bias when mean difference is > 0 and a negative bias when mean difference is < 0. If there is a difference between the results that is constant over all values, then this is considered ____ bias. If there is a difference between the results that changes proportionately with manigtude of the result, this is considered _____ bias.

Answer 67

constant

proportional

Question 68

Describe the graphs in a Deming method comparison and passing bablock method comaparison

How do you interpret the graphs?

Answer 68

Values obtained form new method - y axis

Values obtained from old method or comparison method - x axis

Equation is formed for the best fit line (y = mx + b) and the 95% confidence intervals for the slope and y-intercepts are calculated.

Proportional bias = detected if 95% confidence interval for the slope does not include 1

Constant bias = detected if 95% of the confidence interval for the y-intercept does not include 0

Question 69

What are the main differences between the deming method and passing bablock methods for method comparison?

Answer 69

Both Deming method and Passing Bablock method allow for imprecision, however the deming method requires impercision to be normally distributed (aka parametric analysis) while the passing bablock method does not require that the variances int he data sets have normal distributions (aka it is a nonparametric method).

Question 70

____ is a method comparison that can be used when laboratory results are in a categorical scale, such as 1+, 2+, or trace, mild, moderate, marked. A 2+ value may not be twice that of a 1+ and a 4+ may not be twice the value of a 2+.

Answer 70

Kappa agreement

Question 71

These are distribution curves of healthy and diseased animals.

1. Which graph has the best decision threshold?
2. Which one would have a diagnostic sensitivity of 100%?
3. Which one woul dhave a diagnostic specificity of 100%?

Answer 71

1. C
2. A (b/c there are no false negatives)
3. B (b/c there are no false positives)

Question 72

ROC curves display the relationship between a TP rate and a FP rate. What are TP rate and FP rate equal to?

Answer 72

• TP rate = diagnostic sensitivity expressed as decimal
  
  • when diagnostic sensitivity is 90%, TP rate is 0.9 - results are psotivie in nine of ten diseased animals
• FP rate = 1 - diagnostic specificity expressed as a decimal
  
  • WHen diagnostic specificity is 70%, the FP rate is 1-.07 = 0.3 - 3/10 nondiseaed animals woudl have a FP result

Question 73

How do you evaluate a ROC curve?

Answer 73

The more the curve is to the top left, the better becuase nearly all results are TP - and it means the test is more accurate. You can also look at it as area under the curve, and the more AUC, the better.

Question 74

At what point along the ROC curve would you use use for a decision threshold if you want the best balance between diagnostic sensitivity and specificity?

At what point woudl you use for a decision threshold if you want the test to screen for a diseae?

At what point woudl you use for a decision threshold if you want the test to be a confirmatory test?

Answer 74

1. 40
2. 20
3. 50

Question 75

What bias is being illlustrated by this Bland-Altman Difference Plot?

Answer 75

Constant Bias (all differences below zero)

AND

Proportional bias (trend where differences get more negative as Hgb concentration increases)

*side note: mean difference only tells you about constant bias, it does not take proportiona bias into consideration