Concepts in Laboratory Medicine

Question 1

Why do we need labs

Answer 1

Why do we need labs
-60-70% of decision making in med is done based on results of lab tests*
-aid in early dx of disease that may not have clinical presentation
-monitoring progress of disease

Question 2

why not labs?

Answer 2

why not labs?
-insufficient understanding of labs can lead to misinterpretation of results
-inefficient selection could increase healthcare costs

Question 3

reference range

Answer 3

-Normal range -can differ by facility
- Determined by individuals without disease on no meds
-Middle 95% is reference range
-some people have a abnormal number that is normal for them!
-regional differences- ex. hmg

Question 4

desirable ranges

Answer 4

desirable ranges
-prognosis related ranges
-established by experts
-associated lab results with clinical outcome
-the range that defines certain dx (not certain!)
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Question 5

therapeutic ranges

Answer 5

therapeutic ranges
-used to monitor medication effects
-window for levels
-below -> is typically inadequate level of medication
-above -> toxic effect level of mediation
-not always drug level -> can be effect produced
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Question 6

threshold

Answer 6

Has no range, presence of disease above a level
-ex. troponin, drugs
-once you hit upper limit of normal = confirmation the event is happening -> positive result!
-threshold separates neg and pos results
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Question 7

sensitivity

Answer 7

sensitivity
-identifying population with disease
-capacity to identify all individuals with disease
-however as threshold if lowered to capture all with disease -> false positives can rise
-reliability
-99% sensitivity- if person has condition test will pick it up 99% of time
-(true positives / true positives + false negatives) x 100
-you get more false positives when you increase sensitivity*** -> lowering the bar to capture all the positives but you start including false positives
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Question 8

specificity

Answer 8

specificity
-focus on population without disease
-correctly identify those without disease
-as threshold raised to capture all those without disease -> false negatives can rise
-(true negative/true negatives + false positives) x 100
-you get more false negatives when you increase specificity*
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Question 9

appropriate value

Answer 9

Established to minimize total number of false positives + false negatives**

-HIV- max sensitivity (blood donor) -> this needs to be sensitive bc you need to know if someone has HIV
-pancreatic cancer- max specificity (before treatment) -> you dont want to treat someone if they dont have it

Question 10

positive predictive value

Answer 10

positive predictive value
-indicates the likelihood that positive test result identifies someone with the disease

The proportion of positive test results that are true positives (i.e., the probability that subjects with a positive screening test truly have the disease).

ex:
100 people tested positive for the disease.
Of those 100, 80 actually have the disease (True Positives)
20 = false positive
80/(80+ 20) = 0.8

Question 11

negative predictive value

Answer 11

negative predictive value
-indicates the likelihood that a neg test result identifies someone without disease
-(true negatives/true negatives + false negatives) x 100
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Question 12

prevalence vs incidence

Answer 12

prevalence vs incidence
-prevalence- number of existing cases in population
-incidence- number of new cases occurring within a period of time
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Question 13

precision vs accuracy

Answer 13

precision vs accuracy
-precision- ability to test 1 sample and repeatedly obtain close results - not necessarily the correct but consistent
-accuracy- relationship between number obtained and true result -> correct
-precision and accuracy- consistent and correct
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Question 14

phases of lab analysis

Answer 14

phases of lab analysis
-preanalytical (MC error)- any actions or factors involved in acquiring, handling, transporting, and processing a pt specimen prior to actual analysis
-analytical- all factors related to test platform and to testing process itself
-post analytical- interpretation of the test results in light of our expertise as physicians to formulate a dx (or diff dx) to guide pt management
-errors can happen at every phase
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Question 15

preanalytical errors

Answer 15

Inappropriate prep of patient
Not fasting, ingesting drugs that interfere, wrong tube, delayed transport of specimen, stored at wrong temp, inadequate amt of blood

Question 16

What are types of Analytical errors?

Answer 16

Incorrect use of instrumentation or expired reagents

Question 17

What are types of Post Analytical errors?

Answer 17

reporting results to wrong patient, delayed in time to enter completed results

Question 18

preanalytical variables

Answer 18

-Age
-Gender: Testosterone and estradiol hormones
-Body mass- muscle mass, creatine kinase, serum cholesterol w obesity +body fat
-Prepping patient for lab testing - fasting
-Patient posture for blood collection :lower plasma when pt is upright,
but… when supine theres fluid movement into circulation, extra volume in circulation can dilute certain compounds in blood
- Samples of venous arterial and capillary- diff concs than eachother

Question 19

What are 3 major analytical interferences in lab testing?

Answer 19

-hemolysis -> can cause high K
-bilirubin -> yellow tint
-lipemia -> cloudy serum
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Question 20

drug impact

Answer 20

drug impact
-you must know if people are on drugs
-ex. coagulation study with pt on thinners
-interfering with test
-producing effect on body
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Question 21

selecting a test

Answer 21

selecting a test
-screening before advanced…$$$
-test with reflex
-dont order too many -> remember 5% fall out of normal range
-abnormal results lead to more tests
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Question 22

examples of screening tests

Answer 22

examples of screening tests
-rapid strep test
-urine preg test
-mammogram
-PSA
-pap smear
-total cholesterol level
-TSH
-urine tox screen
-monospot
-COVID home test
-COVID PCR
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Question 23

specimens

Answer 23

specimens
-turn around time
-tube for collection
-timing
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Question 24

blood cultures

Answer 24

-collect 2 samples from diff parts of body bc of possible contamination
-collected before antibiotics
-do one aerobic culture and one anaerobic
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Question 25

types of lab samples

Answer 25

types of lab samples
-blood (peripheral or central)
-urine
-stool
-mucus/sputum
-wound
-tissue
-CSF
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Question 26

prior to obtaining sample

Answer 26

prior to obtaining sample
-pt appropriately identified
-specimen container labeled
-source of quantity must be determined
-verify prior preparation (fasting)
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Question 27

blood samples

Answer 27

-capillary -> heel (newborn) , point of care diabetic testing
-venous -> whole blood, plasma, serum
-arterial

Question 28

Blood that has NOT been clotted and is then centrifuged to remove any cells is known as

Answer 28

PLASMA
-plasma is liquid in spun down tube
-Testing for blood cell counts and clotting factors

Question 29

CLOTTED blood that is centrifuged to remove the clot and any cells is known as

Answer 29

SERUM
- serum is the liquid that remains after the blood was clotted AND the clot was removed -> it has NO CLOTTING FACTORS OR FIBRINOGEN

“think serum + skincare -> its smoother and has no clotting factors because you dont want to clog your pores when you put it on”

Question 30

cell injury

Answer 30

Plasma Markers of Organ Damage:
-Injured cells leak components “markers”
-Troponin:organ damage
-ALT / AST
-not always present -> only shows up when organs are “screaming”

Acute phase reactants:
-Plasma protein changes with inflammation
-ESR, fibrinogen, c-reactive protein, etc

Infections:
-Identify antibody.
-IgM (acute), IgG (Lymes Disease)
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