Introduction to Clinical Chemistry

How close is the test to the true value

How close are repeated measurements to one another

Is the response proportional to the analyte

Is the test affected by other factors such as haemolysis, icterus or lipaemia haemolysis = red tint Icterus = excessive bilirubin Lipaemia = excessive lipids

What is the smallest amount that you can measure

Does the method measure the correct analyte e.g. if on steroids cortisol will be high and won’t indicate cushings disease

(sum of xi)/n

Square root of (sum of (x-xbar)^2/n-1)

Standard deviation/mean x 100

A set of values obtained from a group of individuals of defined health status

Age Gender Weight Diet Pregnancy Menstrual Status Time of day Time of year

Action limits Therapeutic ranges

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

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

Biological variation Pre-analytical variation Analytical variation Post-analytic variation

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

Interindividual Intraindividual

A patient values compared to a population mean

A patient compared to themselves Usually smaller variation than interindividual e.g. bilirubin drops during the day

Combining multiple tests to address a clinical question e.g. lipid profile or liver profile

No overlap between values from healthy and diseased populations but this very rarely happens

Generates some false positives and negatives

Arbitrary value that marks the point between negative and false e.g. in pregnancy levels double every 24 hours

Frequency of positive test result when disease present