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From Flashcardlet > Clinical Lab Med 1 > Flashcards

Clinical Lab Med 1 Flashcards

1
Q

What three things should you consider when ordering laboratory tests?

A
  1. In what situation is the test diagnostic versus when the test provides info without being diagnostic?
  2. What other tests are available? When should you use one test rather than the other?
  3. What are disadvantages of the test and possibilities of error or false results?
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2
Q

What is an example of when a test gives information, but is not diagnostic?

A

CBC - it gives information but is not diagnostic. The patient may have low hemoglobin, but are they anemic? bleeding? bone marrow disorder?

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3
Q

When should you use one test in preference to another?

A

Considering the cost and comfort to the patient

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4
Q

What is the gold standard?

A

The best possible test used to diagnose a patient’s condition

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5
Q

Why don’t we always use the gold standard?

A

It might be too expensive, invasive, or potentially dangerous for the patient

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6
Q

What is an example of when we don’t use the gold standard?

A

A pulmonary angiogram is the gold standard for a pulmonary embolism, but we use a CT instead because it is less invasive and costly and gives almost the same results

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7
Q

What is an important thing to keep in mind when looking at a lab and deciding what to prescribe?

A

We cannot knee jerk react and diagnose and prescribe just based on labs

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8
Q

What is BNP?

A

brain natriuretic peptide - natural diuretic is released to ventricles any time there is stress, therefore increasing the BNP value. supposed to identify CHF

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9
Q

Why can’t we diagnose CHF from SOB and increased BNP?

A

Because many other disorders cause increased ventricular pressure, like PE, renal failure, etc. (if we thought it was CHF and ordered a diuretic for a patient that had PE we would kill them!)

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10
Q

Why is it important to avoid unnecessary testing?

A
  • Financial and personal inconvenience to patient
  • Misleading information
  • Wasted time
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11
Q

What is the chance of a panel of 12 tests bringing back an abnormal value?

A

46% chance of bringing back an “abnormal” value on a healthy patient

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12
Q

What emerged in the 1960s? What was discovered in the 1970-80s?

A

In the 60s, many multichannel chemical analyzers emerged which could make it possible to order large numbers of tests and get results quickly, but we soon realized that few of these tests were useful for screening asymptomatic patients

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13
Q

When should test be ordered?

A

To confirm a diagnosis that is proposed based on the history and physical presentation of the patient, and if the result of the diagnostic test would alter therapy decisions

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14
Q

What will 90% of diagnoses be determined from?

A

Medical history and physical exam

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15
Q

What are labs useful for if we can’t definitely say the patient has a condition based on the lab value?

A

We can rule out that condition if lab values don’t indicate its presence

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16
Q

When wouldn’t you order an angiogram for a patient?

A

When the patient isn’t a candidate or does not consent to having surgery - if the angiogram comes back abnormal, the patient won’t undergo treatment anyways

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17
Q

What is sensitivity?

A

the ability of the test to detect patients with some specific disease

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18
Q

What is specificity?

A

describes how well the test abnormality is restricted to those persons that have the disease in question

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19
Q

What is a false negative?

A

A test sensitivity of 90% for a disease indicates that in 10 % of patients with the disease, the test will not detect it

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20
Q

What is a false positive?

A

A specificity of 90% for a disease indicates that 10% of the test results suggestive of the disease are in fact not due to the disease

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21
Q

Is BNP highly sensitive or specific for CHF?

A

Highly sensitive for CHF (not very specific because an increased BNP would present in other cases as well)

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22
Q

The more specific a test is, the…

A

The more specific a test is, the less sensitive it is, and vice versa

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23
Q

What is prevalence?

A

The incidence of the disease (number of people with the disease) in the population being tested

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24
Q

What is predictive value?

A

Illustrates the fact that the smaller the number of persons with a certain disease in the population, the lower will be the proportion of persons with an abnormal test result who will be abnormal because they have the disease in question (greater proportion of false positive results)

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25
Q

What can a predictive value be applied to any laboratory test for?

A

To evaluate the reliability of a positive or negative result

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26
Q

What must you know before determining the predictive value?

A

The sensitivity and specificity of the test in question

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27
Q

If it is not possible to detect all or nearly all patients with a certain disease, what will not be possible?

A

Impossible to provide a truly accurate calculation of sensitivity, specificity, or predictive value for tests

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28
Q

What is reliability of laboratory tests affected by?

A

Technical performance within the lab

  • experience of technicians
  • reagents involved
  • equipment
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29
Q

What is reproducibility?

A

The measure of how closely the laboratory can approach the same answer when the test is performed repeatedly on the same sample

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30
Q

What is the coefficient of variation?

A

The laboratory converts the standard deviation figure to a percentage of the mean value

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31
Q

What values deem a lab with excellent reproducibility?

A

A coefficient of variation in the range of +/-4% (1 standard deviation - 68% of the values)

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32
Q

What values deem a lab with acceptable reproducibility?

A

A coefficient of variation in the range of +/-8% (2 standard deviations - 95% of the values)

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33
Q

Since in a sample of “normal” test results includes 2 standard deviations, what does this mean about the testing sample that lies outside of these values?

A

5% of the testing sample that was “healthy” when tested lie outside of the “normal” range (therefore if your patient falls just outside of the “normal” range, it does not mean that they are unhealthy or “abnormal”

34
Q

What term should be used instead of “normal range”?

A

Reference range

35
Q

What does good reproducibility NOT ensure?

A

Accuracy

36
Q

The higher the coefficient of variability, the (better/worse) the reproducibility?

A

Higher coefficient of variability, worse the reproducibility (therefore, less chance for accuracy)

37
Q

What is accuracy?

A

The correct answer, or the result or value that the assay should produce

38
Q

What does the lab use as a control?

A

The lab analyzes control specimens with each group of patient specimens so if a technical factor produces erroneous patient results it will result in erroneous control results as well

39
Q

What factors might influence the results of the patient specimen but not the control specimen?

A
  • specimen from wrong patient
  • specimen hemolysis
  • insufficient mixing
  • clerical errors - patients with same last name
  • decimal point mistakes
  • transcription of results into wrong chart
40
Q

What should be kept in mind when encountering unexpected lab abnormalities?

A

Keep the context of the clinical picture in mind when experiencing unexpected results

41
Q

What should be done if unexpected results are received?

A

The test should be repeated, especially if the results were to call for extensive work up or therapeutic intervention

42
Q

What two premises involve establishing a normal range?

A
  1. Assume that all persons that do not demonstrate clinical signs and symptoms are normal
  2. Assume that test results from those persons considered normal will have a random distribution
43
Q

What percentage of values fall within 1 standard deviations?

A

68%

44
Q

What percentage of values fall within 2 standard deviation?

A

95%

45
Q

What percentage of values fall within 3 standard deviations?

A

99.7%

46
Q

What are some problems associated with the use of normal ranges?

A
  1. A small group of clinically normal person may have subclinical or undetected disease and may be inadvertently included in the supposedly normal group
  2. Normal ranges are sometimes calculated from a number of values too small to be statistically reliable
  3. Various factors may affect results in a non diseased person
47
Q

What happens if a person with the disease is included in the range of clinical normal values?

A

The abnormal person’s values will now be considered normal, the the limits of the reference range may be influenced

48
Q

How would we make sure that the person with the disease is not included in the reference range values?

A

Use the gold standard to test the person

49
Q

What various factors may affect results in a non diseased person?

A
  • age
  • sex
  • race
  • diet
  • posture
  • specimen storage time
50
Q

The greater the number of tests employed, the (lesser/greater) the chance that at least one will idld a falsely abnormal result

A

Greater

51
Q

What can reinforce an abnormal result on a panel?

A

If there are three values related to liver proteins that are all abnormal, that is a good indication that something is wrong.
If one liver value is abnormal and the other two are in the reference range, question it

52
Q

When may abnormal test results occur in a healthy patient (general)?

A

When the patient’s physiologic state changes

53
Q

What are some examples of when a patient’s physiologic state changes test results? (5)

A
  • hormonal and metabolic changes following food absorption
  • stress
  • exercise
  • cyclic variation
  • medication
54
Q

What happens to lab results after a meal in terms of gastric acid production? (4)

A
  • Meals directly stimulate production of gastric acid,
  • leading to loss of HCl,
  • increasing pH of blood,
  • lowering plasma chloride concentration
55
Q

What happens to lab results after a meal in terms of glucose absorption? (4)

A
  • Glucose absorption stimulates insulin release,
  • producing rise in plasma levels,
  • then insulin facilities entry of potassium and phosphate into cells,
  • causing drop in plasma levels after eating
56
Q

What may non-fasting lab samples result in?

A

Abnormal results

57
Q

When are the most reproducible lab results collected?

A

After a patient has been fasting for 8-12 hours, since they are more likely to correlate with the actual physiologic state

58
Q

How does stress effect lab results?

A

Changes in plasma volume accompany acute stress

This dilutes plasma proteins due to increase in blood volume

59
Q

How does exercise effect lab results?

A

Patients who exercise regularly have increased levels of plasma enzymes found in muscle:

  • creatine kinase (CK)
  • aspartate aminotransferase (AST)
  • lactate dehydrogenase (LDH)
60
Q

What misdiagnosis can be made when looking at lab results of a patient who exercises regularly and intensely?

A

Lives disease, because many of the increased plasma enzymes found in their labs are found in the liver

61
Q

How drastic can CK changes be in a patient who exercises? What does this make it difficult to monitor?

A

CK changes can reach 10-20 times the upper limit of the reference range, making it difficult to follow the effects of cholesterol lowing medications (statins) and their effects on muscle breakdown and rhabdomyolysis

62
Q

What is cyclic variation in lab testing?

A

Reproducible patterns of alternation in physiologic state occur at specific times of the day, month, or year

63
Q

What is a time of cyclic variation in females?

A

Menstruation

64
Q

When in cholesterol higher in females than usual?

A

Cholesterol is 25% higher in the second half of the menstrual cycle

65
Q

What factors can produce a non-representative specimen?

A

Usually related to technique of collection:

  • patient preparation
  • tourniquet use
  • contamination with IV fluids
  • contamination with anticoagulants
66
Q

How does patient preparation effect a lab sample?

A

When the patient is in the upright position, water and electrolytes are lost from vascular space, resulting in proteins and protein bound substances becoming relatively concentrated
When the patient goes from a sitting to supine position, water and electrolytes move back into vascular space, reducing the concentration

67
Q

What compounds may be increased in concentration if the patient is in the upright position?

A
  • plasma proteins
  • enzymes
  • hematocrit
  • calcium
  • iron
  • hormones
  • drugs
68
Q

What is the increase in concentration of compounds in the upright position, on average?

A

Increase of 5-8%

69
Q

What is the decrease in concentration of compounds in the supine position, on average?

A

Decrease of 10-15%

70
Q

What two factors effect a sample with tourniquet use?

A
  • loss of fluid

- leakage of cell contents

71
Q

How does loss of fluid while using a tourniquet impact the lab sample?

A

Venous pressure remains high, and the return of water and electrolytes to venous blood from the tissue is impaired. This results in increased concentration of proteins and protein bound substances

72
Q

How long does a tourniquet have to be on to increase the concentration of proteins in the sample by 5%?

A

3-5 minutes

73
Q

How long does a tourniquet have to be on to increase the concentration of proteins in the sample by 15%?

A

10-15 minutes

74
Q

How does leakage of cell contents while using a tourniquet impact the lab sample?

A

Venous pressure remains high, and circulation of fresh blood diminishes. Cells continue their metabolism, resulting in a fall in the concentration of substances used in metabolism, and increasing the concentration of metabolic products such as lactate

75
Q

What does increased lactate accumulation result in?

A

Acidosis

76
Q

What does acidosis lead to?

A

Leakage of potassium from cells, resulting in a factitiously elevated serum potassium

77
Q

What can increase the rate at which metabolic changes occur?

A

Clenching and unclenching the fist

78
Q

How can a specimen be contaminated with IV fluids?

A

Obtaining the specimen from a vein that is receiving IV fluids can result in diluted values

79
Q

Where should the tourniquet be placed if there is no other option than to use an arm that is receiving IV fluids?

A

Tourniquet should be placed between the IV and phlebotomy site

80
Q

What sample goes in a lavender top tube?

A

CBC

81
Q

What sample goes in a red top tube?

A

Metabolic panels, lipid profiles, thyroid function tests

82
Q

What sample goes in a blue top tube?

A

Coagulation studies (PT, INR, PTT)

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