Lab Tests Flashcards
Why are blood tests done?
largely to monitor drug handling.
-other health professionals can use to diagnose. (E.g. Hb1AC is a measure of glycylastion in haemoglobin.)
-Can use tests to decide drug treatments. ◦E.g. Warfarin:
-Check patient drug use. e.g. lipid levels for patients on statin.
•We can monitor drug levels.
What are the limitations with using blood tests?
-Variation between individuals for the same condition,
-variation within an individual over time. (So looking at an absolute value does not tell you too much, need to look at the big picture.)
-Look at trends over time and the normal range.
- Consider the quality of the test, how sensitive it is. (If the disease exists is the test sensitive enough to pick it up)
And specificity, if the test says you have the disease, you actually have it
-Consider other things which might affect the result you see.
What are some compounding factors with lab tests?
Unless you request serum potassium levels pre-haemolysis, the lab technicians will haemolyse the blood, resulting in a higher serum potassium which does not accurately reflect patient K levels
May get mechanical shear during the withdrawing process.
What you see on the lab sheet may not tell you what is going on with the patient
What is sodium and osmolality involved in ?
muscle function and various other things, but mostly to do with water and cell tonicity
What are possible causes for very low serum sodium and normal potassium?
poor renal function
dilutional hyponatriuaemia
what is dilutional hyponatriuaemia?
you have the same amount of sodium, but it looks lower because it has been diluted by water.
What is SIADH?
syndrome of inappropriate diuretic hormones.
Caused by some SSRI
Why is a high K level concerning?
Potassium is mainly intracellular so you shouldn’t see a lot in serum
Potassium is related to the heart and affects the force of contraction, electric activity and other muscle function so you want a fairly tight control of id.
Which part of the RAAS system is responsible for potassium?
aldosterone. A lot of drugs can cause potassium sparing.
What is hyperkalaemia also suggestive of?
renal impairment, seen by an accumulation of a lot of ions. Treat quickly
What ions are most commonly lost in vomiting?
sodium and potassium
What ions are most commonly lost in diarrhoea?
Potassium
Would a patient lose ions if they are dehydrated?
probably not a lot
What ion would be lost if patient has a fever or are sweating?
a lot of sodium
What does giving a patient fluids correct?
sodium and potassium levels
what is the relationship between potassium nad insulin?
you do need potassium to be able to utilise insulin.
which drugs can be used to monitor potassium?
insulin and salbutamol.
Insulin causes potassium to be absorbed into cells, and can be used for hypokalaemic patients
while salbutamol causes potassium to be excreted from cells and can be used for hyperkalaemic patients
What is part of a FBC?
A full blood count involves,
- amount of RBC
- amount of WBC and which types there are (differential)
- thrombocyytes
What does a WBC differential tell us?
If someone has a viral or bacterial infection
What is a FBC useful for?
diagnosing anaemia, leukaemia, but it would not tell us about clotting abnormalities
How are clotting abnormalities detected?
by drawing blood and timing how long it takes to clot
What do liver function tests tell us?
These do not measure liver function, but they can tell us a lot about liver injury
Is the liver involved in thermoregulation?
yes
What is bilirubin?
The breakdown product of RBCs after conjugation in the liver, to make them soluble and able to be eliminated.
The soluble form is urinated out while the insoluble form is excreted through the bile
What does an accumulation of bilirubin indicate?
It may suggest there is a problem with the outflow of bile
What are the components of LFTs?
bilirubin aspartate aminotransferase (AST) Alanine aminotransferase (ALT) gamma glutamyl transpeptidase (gamma-GT) Alkaline phosphatatse (alkphos) Lactate dehydrogenase (LDH)
How are liver abnormalities diagnosed?
Pattern recognition combining the results of all the tests is crucial because individual tests are not specific for liver disease.
assessments should be guided by history, risk for liver disease, duration and severity of clinical findings, any comorbidities, and the nature of the liver test abnormality noted
What are the traditional groups of liver test abnormalities?
hepatocellular (predominantly ALT and AST elevations)
Cholestatic (predominantly alkaline phosphatase elevation)
infiltrative/mixed abnormalities
In what situation might patients with liver disease exhibit normal LFT values?
in patients with hepatitis C (16%)
In patients with varying histological damage due to NAFLD- non alcoholic fatty liver disease) (13%)
In patients with hep B in immune tolerant phase and in the inactive HBsAg carrier state
What is gamma glutamyl transferase?
One of the liver enzymes to break down waste products
Very prone to liver activity (if liver works very hard by drugs, medicines and alcohol)
Used to monitor use of alcohol
What is ALT?
alanine aminotransferase is an intracellular enzymes living inside our liver cells.
This test is NOT LIVER SPECIFIC
If this is abnormal, but all other LFTs are normal, this is not suggestive of liver abnormalities
Why is ALT not liver specific?
It is also found in bones and lungs
Raised ALT + normal other LFTs might indicate bone disease
What is AST?
It is an enzyme found in liver, but also in the heart, and to a lesser extent, the kidneys and muscles.
In liver injury, AST is released into the blood so you get elevated AST (BUT this might not always occur in liver disease)
What does the ratio of AST:ALT tell us?
If AST>ALT, this indicates alcoholic hepatitis and is rarely seen in drug induced liver disease
The higher the ratio, the more likely it is to be alcohol related.
e.g. if its >2, this means they are usually a drinker, if it is 1, then they are likely to be non-alcoholic.
Of ALT and AST, which is more specific to the liver?
ALT. hence injury to the liver = more alanine in the blood.
NB: people who run marathons may have slightly elevated ALT
What is liver injury?
hepatocellular damage. cholestatic disease (to do with the bile duct)
What does a high bilirubin + normal LFTs indicate?
haemolytic disease
What is jaundice?
Bilirubin accumulation which can be seen in the eyes.
This is the breakdown of haemoglobin and myoglobin
What does ALP measure?
bilirubin in the liver caniculi.
Is GGT liver specific?
yes. you see it in all forms of liver disease, i.e. due to cellular damage or cholestatic injury, but it is not very helpful diagnostically.
Can GGT be a sole biomarker of alcohol?
No. It is also elevated as a result of some drugs such as rifampicin
It is very sensitive to change and its response to alcohol is fairly short phased, so it is NOT a great measure for chronic alcoholics.
In what situation would transaminase levels appear very low?
people with chronic hepatitis who, over time, have such few uninjured cells left, the level of transaminases reflect cell injury, but are low due to a low number of healthy cells.
Why are clotting factors measured?
to measure the right dose of warfarin
to measure whether patient is synthesising clotting factors
i.e. a function test
How is renal function measured?
By measuring filtration of creatinine clearance
Why is creatinine clearance used?
In theory, it is not absorbed or secreted, so it is only filtered.
We estimate this using the Cockcroft gault equation
What is the Cockcroft gault equation?
(140-age) x (mass) x constant/ serum creatinine in umol/L
where constant = 1.23 for males
1.04 for females
in what situations would you need to adjust the Cockcroft gault equation?
if a patient has a missing limb, as they have lost muscle mass.
adjust by calculating their lean bodyweight.
What are the 5 main uses of clinical tests?
confirm or support diagnosis assess the severity of disease monitor the response to a treatment monitor appropriate drug dosing help prevent toxic side effects and interactions.
What is accuracy?
the extent to which the mean measurement is close to the true value
What is precision?
the reproducibility of the assay or test
What is sensitivity?
The ability of the test to show positive results in patients who have the disease in question
What is specificity?
the percentage of negative results among people who do not have the disease in question
What are the main factors (not medicines or comorbidities) affecting test results?
reference values differ in different body fluids, leads to confusion and clinicians misusing terminology
Reference ranges differ significantly with various measurement methods
The method of specimen collection, transport, storage and process
The levels of blood constituents fluctuate throughout the day, affecting results.
Age of the patient
Sex of the patient
pregnancy
small effects dues to race/nationality
body state (e.g. fasting)
nutrition state
physical activity
patient position (changes from lying to sitting or standing reduces plasma volume)
