Introduction to Clinical Chemistry Flashcards
Define accuracy
How close is the test to the true value
Define precision
How close are repeated measurements to one another
What is linearity
Is the response proportional to the analyte
Define interference
Is the test affected by other factors such as haemolysis, icterus or lipaemia
haemolysis = red tint
Icterus = excessive bilirubin
Lipaemia = excessive lipids
What is analytical sensitivity?
What is the smallest amount that you can measure
What is analytical specificity
Does the method measure the correct analyte e.g. if on steroids cortisol will be high and won’t indicate cushings disease
What is the equation for finding the mean (accuracy)?
(sum of xi)/n
What is the equation for finding precision/SD?
Square root of (sum of (x-xbar)^2/n-1)
How do you find the coefficient of variation
Standard deviation/mean x 100
What is a reference range
A set of values obtained from a group of individuals of defined health status
List the factors that influence reference ranges
Age
Gender
Weight
Diet
Pregnancy
Menstrual Status
Time of day
Time of year
What can be used instead of reference ranges
Action limits
Therapeutic ranges
What are action limits
means the minimum and maximum values of a quality assurance measurement
If the analyte is higher than the action limit then you need to intervene e.g. paracetamol overdose
What is a therapeutic window
Has a min and max
Need to get patients within this range
If too high there is toxic side effects
If too low the drug is not effective
Where are the four sources of variation in laboratory medicine
Biological variation
Pre-analytical variation
Analytical variation
Post-analytic variation
Give some areas where there is pre-analytical variation
(7)
Type of sample -> venous or capillary
Collection tube
Fasting or not
Specimen handling -> labelling, transport and storage
Strenuous exercise
Unreadable information on sample
Old sample
What are the two ways of biological variation
Interindividual
Intraindividual
What is interindividual variation
A patient values compared to a population mean
What is intraindividual variation
(3)
A patient compared to themselves
Usually smaller variation than interindividual
e.g. bilirubin drops during the day
What is a panel approach
Combining multiple tests to address a clinical question
e.g. lipid profile or liver profile
What is an ideal diagnostic test
No overlap between values from healthy and diseased populations but this very rarely happens
What happens when there is overlap in a test
Generates some false positives and negatives
What is a cutoff point
Arbitrary value that marks the point between negative and false e.g. in pregnancy levels double every 24 hours
What is diagnostic sensitivity
Frequency of positive test result when disease present
What is diagnostic specificity
Frequency of negative test result when disease absent
What is diagnostic specificity
Frequency of negative test result when disease absent
What is the equation for diagnostic sensitivity?
True positives/(true positives + false negatives)
What is the equation for specificity?
True negatives/(true negatives + false positives)
What is positive predictive value
Tells you the odds of you having the disease if you have a positive result
What is negative predictive value
Tells you the odds of you not having a disease if you have a negative result
What is the equation for positive predictive value?
Positive test result with disease + positive test without disease
What is the equation for negative predictive value?
negative test with disease + negative test with disease
What is sensitivity in terms of a question to ask yourself
I know my patient has the disease. What is the chance that the test will show that my patient has it?
What is specificity in terms of a question to ask yourself
I know my patient doesn’t have the disease. What is the chance that the test will show that my patient doesn’t have it
What is PPV in terms of a question
I just got a positive test result back on my patient. What is the chance that my patient actually has the disease
What is NPV in terms of a question you can ask yourself
I just got a negative test result back on my patient. What is the chance that my patient actually doesn’t have the disease
How do you define and determine reference intervals
(7)
Reference individuals
Make up a reference population
From whom are selected a reference sample group
On whom are determined reference values
On which is observed a reference distribution
From which are determined reference limits
That define a reference interval
