Lecture 19 Flashcards
Principles of Biochemical Assessment
forms of nutritional assessment
- anthropometric assessment
- dietary assessment
- biochemical assessment
- clinical assessment
Subclinical deficiency
Before you get any clinical signs
There are two methods that detect subclinical deficiency and confirm clinical diagnosis
- static biochemical tests
- functional tests
what are two static biochemical tests
- nutrient in biological fluids or tissues (e.g plasma zinc)
- urinary excretion rate of nutrient or metabolites (e.g 24 hour urinary iodine)
what are two functional tests of biochemical assessment
functional biochemical tests (e.g glutathione peroxidase activity)
functional physiological or behavioural tests (e.g taste acuity for zinc)
glutathione peroxidase activity is a measure of and how does it work
selenium status,
this works as selenium is a critical component of GP, can look at the rate that GP is able to neutralise a peroxide (as GP is an antioxidant)
Precision
the degree to which repeated measurements of the same biomarker give the same value
how to measure precision in biochemical assessment
repeated measures on pooled sample(s) -> Coefficient of Variation (CV)
Within run and between run CVs
What is meant by within run and between run CVs when measuring precision in biochemical assessment
Within run is within that time you measure and between runs is if you come back and measure the same thing and compare Coefficient of Variation
Analytical accuracy
The difference between the reported and the true amount of the nutrient/metabolite present in the sample is a measure of the analytical accuracy (“trueness”) of the laboratory test
Ways of measuring analytical accuracy in biochemical assessment
- recovery test on spiked samples
- certified reference materials
- analysis of pooled sample by multiple labs using different methods
Example of certified reference materials for analytical accuracy
human hair = dried powdered hair with a certain amount of zinc in it
you would check if your assay was giving you the right amount
Analytical sensitivity
the smallest concentration that can be distinguished from the blank
what is the limit in analytic sensitivity
~ minimum detection limit
values less than the minimum detection limit should not be recorded
Analytical specificity
the ability of an analytical method to measure exclusively the substance of interest
how can analytical specificity be enhanced
by dry ashing or wet digestion
what is dry ashing
Burning the sample which is then used to enhance analytical specificity
what is wet digestion
breaking down all the other organic material used to enhance analytical specificity
Validity
How well the biomarker correctly describes the nutritional parameter of interest.
what is an example of validity
If the biomarker selected reflects recent dietary exposure, but the study objective is to assess the total body store of a nutrient, the biomarker is said to be invalid
Key aspects of validity
- the assay must be appropriate for the study objective
- drugs, hormones, infection may alter laboratory test results but not actual status
What is sensitivity
How good your method is at identifying people who are a concern = like those who had low intakes
what is specificity
How well does the method identify people who are fine
What is the positive predictive value
If positive result of the test, what is the chance that the test is correct
What is the negative predictive value
If negative result of the test, what is the chance that the negative result is correct
What is a venous vs capillary
venous is from the vein
capillary is from the capillary
How to get capillary blood
Heal prick or finger prick
when you squeeze the blood out you are not just getting capillary blood you are also getting (milk the sample)
the fluid as well which dilutes it= can lead to haemodilution
Red top blood collection tubes
This is often used to measure
The blood clots when it enters
Often used when measuring serum ferritin levels
Most of the blood collecting tubes in NZ are
Vacutainers (they have a vacuum in them, supports the blood to fill the tube)
lavender / purple top blood collection tubes have
What would they often be used for
Has EDTA (anticoagulant) added to it, so the blood doesn’t clot
Often used for whole blood count
What are the fancy / gold top blood collection tubes
What tubes would they replace
have gel at the bottom, clot activator and gel separator, has powder that encourages clotting
would replace rep top
what are dark blue/black top blood collection tubes
and what are they important for
trace element free tube, also prevents clotting
zinc is everywhere so it is super easy to contaminate samples, these are important when measuring zinc
what happens to the blood in a purple top
the platelets have been stopped from clotting
what are the key steps in data collection when measuring serum zinc concentration to assess population zinc status
- age
- sex
- time of day
time since last meal - presence of symptoms of infections
- other contributing factors such as oral contraception
when collecting zinc blood samples what is important
draw blood using stainless steel needle and collect into trace element free evacuated blood collection tubes
is plasma and serum often separated in zinc blood samples
yes
analytical accuracy is ideally determined using
certified reference materials
venous and capillary blood do not
give the same results
what is serum
Serum is the liquid that remains after the blood has clotted
what is plasma
Plasma is the liquid that remains when clotting is prevented with the addition of an anticoagulant
