Chem. Lab Rotation (QC and Cardiac)

Question 1

(clinical/diagnostic) Sensitivity vs. Specificity

Definition and calculation

Answer 1

“Diagnostic sensitivity” is the percentage of persons who have a given disorder who are identified by the assay as positive for the disorder.

TP / TP + FN

“Diagnostic specificity” is the percentage of persons who do not have a given condition who are identified by the assay as negative for the condition. False-positive reactions occur because of sample contamination and diminish the diagnostic specificity of the assay.

TN / FP + TN

Question 2

Define positive and negative predictive value.

Answer 2

Predictive value of a positive test (PV+) – the proportion of positive

tests that represent disease:

TP/TP + FP (troponin)

Predictive value of a negative test (PV-) – the proportion of negative tests that represent lack of disease:

TN/TN + FN (D-dimer)

Question 3

Deference between Efficiency and prevalence?

Answer 3

Efficiency – the proportion of tests that correctly classify the patient:

TP + TN / TP + FP + TN + FN

Prevalence – the proportion of the tested population with the disease:

TP + FN / TP + FP + TN + FN

Question 4

Define reference range.

Answer 4

• normal values
• what is most prevalent in a reference group taken from the general population.
• range of values for a physiologic measurement in healthy persons
• It is a basis for comparison (a frame of reference) for a physician or other health professional to interpret a set of test results for a particular patient.

Question 5

How is the following calculated?

median

mode

SD

CV

Answer 5

Median: middle number

Mode: number that appears the most

SD: square root of the variance

CV: (SD/mean) x 100

Variance: [Σ( x_i - mean)²] / n-1 (sample variance) **** just N for population variance

Question 6

T-test (come back to it)

Answer 6

come back to answer later

Question 7

What percentage of a normally distributed population will fall between 1, 2, and 3 standard deviations from the mean?

Answer 7

one SD: 68.2%

two SD: 95.5%

three SD? 99.7%

Question 8

Describe the path of workflow for chemistry samples.

Answer 8

Question 9

Discuss the use of Westgard rules used in QC and describe how they helpdetermine whether a problem concerns accuracy and/or precision.

Answer 9

Series of rules to evaluate control performance

Decision criteria chosen to maximize efficiency

decreased false rejection of assays (Pfr)

increased detection of true error increase (Ped)

Rules differentiate between increases in random and systematic error

Question 10

1-2s Rule

Answer 10

One control exceeds a control limit of : mean + 2s OR mean – 2s

Considered only a warning rule. Failure of this rule alone does not reject the assay.

Question 11

1-3s Rule

Answer 11

One control exceeds a control limit of: mean + 3s OR mean – 3s

In the absence of other violations, it suggests an increase in random error.

Question 12

2-2s Rule

Answer 12

Two simultaneous or consecutive control values exceed the same control limit of

mean + 2s OR mean – 2s

Can apply to both controls in the same run or the same control in consecutive runs

2-2s violation suggests an increase in systematic error

Question 13

R-4s Rule

Answer 13

The range between two control observations within a run exceeds 4s.

Violation of this rule suggests an increase in random error.

Question 14

4-1 s Rule

Answer 14

Four consecutive control observations exceed the same control limit:

mean + 1s OR mean – 1s

Can apply to last four assays of one control or last two assays of two controls.

Violation of this rule suggests a persistent moderate systematic error.

Question 15

10x Rule

Answer 15

Ten consecutive control observations fall on the same side of the mean.

Rule applies to last 10 assays of one control or last 5 assays for both of two controls

Violation of this rule suggests a long-standing systematic error

Question 16

Discuss the use of LDH isoenzyes in determining whether myocardial infarct has occurred and if so when the injury occurred.

Answer 16

Lactate dehydrogenase catalyses the conversion of pyruvate to lactate. LDH-1 isozyme is normally found in the heart muscle and LDH-2 (muscle/liver) is found predominately in blood serum. A high LDH-1 level to LDH-2 suggest MI. LDH levels are also high in tissue breakdown or hemolysis. It can mean cancer, meningitis, encephalitis, or HIV. This test isnt as specific as Troponin.

******TEST IS NOW OBSOLETE (ELECTROPHORESIS :(Separate by electrophoresis, overlay with substrate, visualize by fluorescence or dye. LD-1 > LD-2 (“flip”) suggests myocardial origin)

Question 17

Discuss the use of CK-MB in determining whether myocardial infarct has occurred and if so when the injury occurred.

Answer 17

MB = “myocardial band”

100% sensitivity by 6-8 hours; returns to normal by 2-3 days

Relatively good cardiac specificity, but since it has a short duration, it cannot be used for late diagnosis of acute MI but can be used to suggest infarct extension if levels rise again.

M and B subunits from separate genes:

MM dominant in skeletal muscle

MB “specific for myocardium”

BB in smooth muscle and brain

***Sandwich assay with anti-M and anti-B antibodies

Question 18

Discuss the use of Myoglobin in determining whether myocardial infarct has occurred and if so when the injury occurred.

Answer 18

Muscle (skeletal and cardiac) O2 carrier

Most rapidly rising marker 85% sensitivity by 3 hours, 100% sensitivity by 6 hours

Non-specific for myocardial source

Returns to normal by 12-24 hours

Elevated in renal disease (↓ clearance)

Question 19

Discuss the use of Tropinin determining whether myocardial infarct has occurred and if so when the injury occurred.

Answer 19

Not present in smooth muscle

TnC – Calcium-binding function (18 kDa) (skeletal muscle also)

TnI – Inhibition of ATPase (22 kDa)

TnT – Tropomyosin-binding (37 kDa)

The most sensitive and specific test for myocardialdamage. Because it has increased specificity compared with CK-MB, troponin is a superior marker for myocardial injury.

Troponin is released during MI from the cytosolic pool of the myocytes. Its subsequent release is prolonged with degradation of actin and myosin filaments. Isoforms of the protein, T and I, are specific to myocardium. Differential diagnosis of troponin elevation includes acute infarction, severe pulmonary embolism causing acute right heart overload, heart failure, myocarditis. Troponins can also calculate infarct size but the peak must be measured in the 3rd day. After myocyte injury, troponin is released in 2–4 hours (detectable at 4) and returns to baseline within 5-10 days.

Question 20

Discuss the use of BNP determining whether myocardial infarct has occurred and if so when the injury occurred.

Answer 20

This is increased in patients with heart failure. It has been approved as a marker for acute congestive heart failure. Pt with < 80 have a much higher rate of symptom free survival within a year. Generally, pt with CHF will have > 100.

B-type natriuretic peptide (BNP): Secreted by myocardial cells in response to LV stretch or stress

(proBNP) cleaved to: BNP – 32 aa active molecule (20 min half life): Vasodilatation, Natriuresis – renal sodium excretion

NT-proBNP (1-2 hr half life) – 76 aa inactive fragment: Net effect is to decrease resistance to pumping by decreasing blood volume (natriuresis) and pressure (vasodilatation). Mostly renal clearance. Concentrations 3-4x BNP.

Question 21

Discuss the use of CRP in determining whether myocardial infarct has occurred and if so when the injury occurred.

Answer 21

Acute phase reactant produced by the liver in response to injury, infection or inflammation. Non-specific but sensitive marker of coronary inflammation (in otherwise healthy patients). Has prognostic value for coronary artery disease and subsequent coronary syndrome. (High CRP (high sensitive assay) may=future cardiovascular events/stroke.

_{< 1.0 mg/L = low risk, 1.0– 3.0 mg/L = average risk, > 3.0 mg/L = high risk}

_{Repeat in ≥ two weeks and average for risk}

_{>10.0 mg/L = probable gross inflammation}

_{Repeat when “metabolically stable”}

Potential confusion:

“Standard” assay units are mg/dL

^{<strong>Neonatal infection surrogate marker</strong>}

^{<strong>Post-surgical healing</strong>}

****Measure immunologically by nephelometry or turbidimetry

Question 22

Describe how homocysteine can be used to predict risk of myocardial infarct.

Answer 22

Excess Homocysteine is related to higher risk of CHD, stroke, and peripheral vascular disease.

Excess Homocysteine in the blood can cause injury to arterial vessels due to it’s irritant nature. This can culminate in inflammation, plaque formation and lead to blocked blood flow to the heart.

Inconsistency between Hcy levels and actual coronary vascular disease in studies has led to elevated Hcy levels NOT being MAJOR risk factor for CVD.

Question 23

Discuss the significance of results from a lipoprotein a (Lpa) test.

Answer 23

The structure of lipoprotein (a) is similar to plasminogen and tPA (tissue plasminogen activator) and it competes with plasminogen for its binding site, leading to reduced fibrinolysis. Also, because Lp(a) stimulates secretion of PAI-1, it leads to thrombogenesis. Lp(a) also carries cholesterol and thus contributes to atherosclerosis. In addition, Lp(a) transports the more atherogenic proinflammatory oxidized phospholipids, which attract inflammatory cells to vessel walls and leads to smooth muscle cell proliferation.

High Lp(a) predicts risk of early atherosclerosis independently of other cardiac risk factors, including LDL. In patients with advanced cardiovascular disease, Lp(a)indicates a coagulant risk of plaque thrombosis. Apo(a) contains domains that are very similar to plasminogen (PLG). Lp(a) accumulates in the vessel wall and inhibits binding of PLG to the cell surface, reducing plasmin generation, which increases clotting. This inhibition of PLG by Lp(a) also promotes proliferation of smooth muscle cells. These unique features of Lp(a) suggest Lp(a) causes generation of clots and atherosclerosis.

Question 24

Draw a diagram that shows how cardiac markers rise and fall after a myocardial infarct.

Answer 24