Interpretation of Lab Results Flashcards
What is the diagnostic cycle?
Health problem
History and Exam
Test request and blood
Analysis
Test results
Interpretation
What do you need to consider when collecting blood?
Dietary Restrictions - fasting - glucose, cholesterol and triglycerides.
Timing - duirnal variation - cortisol, testosterone, Therapeutic drug monitoring (time form last dose), some things are unstable.
Affected by venous stasis
Posture
what are the different collection additives and tubes used?
Clotted blood:
White - plain serum no gel
Brown - plain serum gel
Clot quickly
Orange - Lithium Heparin (anticoagulant)
Red -Potassium EDTA (anticoagulant) - normal blood count
Green - Tri-sodium citrate (anticoagulant) - used for clotting studies
Yellow - sodium flouride?potassium oxalate (anticoagulant) - can tell glucose
what is the reference population?
A healthy populations meausurements. Consider age, gender ethnicity.
Plot a graph.
How would you interpret under the graph?
The mean is the middle of the graph
One standard deviation above and below the mean shows 67% of people’s measurments
Two standard deiviations is 95%
Three standard deviations is 99%
How many people fall in the reference range?
95%
How many healthy people fall outside the reference range?
1 in 20
Are some people who fall outside the reference range still healthy?
Yes
What shows a skewed distribution?
TSH
What would happen if you altered a skewed distripution reference range to a gaussian reference range?
Millions of more people would have to undergo tests and this would be expensive
What are the guidling of sub-clinical hypothyroidism?
If TSH is above 10 then have thyroxine unless pregnant
Why could the TSH guidlines be an issue?
Every lab measures TSH slightly differently so if you get your blood test in one lab you might not get thiroxin but you might if you go down the road
what affects reference ranges?
Precision - How reproducable is this result?
Accuracy - How near to the true value is this result?
what happens to the reference range when you have poor precision?
It widens
What happens to the reference range when its inaccurate?
It moves the reference range
Why wouldn’t you give a reference range and an example of this type of molecule?
When the normal population have results which are associated with the disease e.g Cholesterol as there is no safe level of this.
What two things could cause a result to change?
Analytical variation
Biological variation (within a day or between a day)
How can you tell what change in result is significant to the clinicians?
Take into consideration both analytical and biological variation. There are tables for this.
You normally just monitor people and see if they get worse or better
Is there normally overlap between healthy and diseased populations?
Yes
What are cut offs and clinical sensitivity and specificity?
This at what point you say someone is ill.
If you put the cut off at the end of the reference range then you will have an increase in false negative (poor clinical sensitivity) but good clinical specificity (few false positives).
If you put the cut of at the beginning of the disease range you will get poor specificity (many false posititives) and good sensitivity few false negatives).
Screening tests - when deciding cut offs what do you need to know?
The prevalence of the disease in the populaiton
What does prevalence mean?
Number of people in the population with the condition
When doing a screen for a rare disease what do you need and why?
extremely high specificity and sensitivity as you are screening healthy people
What are examples of rare diseases everyone is screened for?
Congenital hypothyroidism or PKU
How many neonates tested for PKU and congenital hypothyroidism would be a false positive?
For every one neonate affected 25 would be a false positive
What do you do if you are getting lots of false positives in screening?
change the cut off
Errors in lab testing - Pre analytical - what is the common failures?
Choose correct tests in the correct manner on the correct patient
what is a common disease people are diagnosed with because of error?
hyperkaelemia
Analytical errors?
Is the test appropraite for clinical needs is there any interferance.
What is a common interferance in immunoassays and how do you deal with this?
Endogenous antibodies and you can treat this to get rid of it.
What happens in heterophilic antibody interferance?
In an immunoassay the heterophilic antibody binds onto the antibodies that are meant to be there stopping interactions.
What happens to an immunoassay which is seeing High dose hook - false negative?
The antibodies are overwhelming the system and there is not enought labelling antibody leading to binding of analyte but no signal so it is not seen.
What would happen is there was an incorrect result which said someone has high hCG?
Unnecessary chemo and hysterectomy
What would happen is there was an incorrect result which said someone has low digoxin?
Overdose leading to toxicity
What would happen is there was an incorrect result which said someone has low insulin?
confusion over diagnosis of insulinoma
What would happen is there was an incorrect result which said someone has high troponin?
Incorrectly diagnosed MI
Post analytical errors?
Communicating the right results
Getting the interpretation right
