Diagnostics Flashcards
Which anticoagulants are present in the tubes with red, yellow, purple, grey and blue tops?
Red top = no anticoagulant Yellow top = gel (procoagulant) Purple top = potassium EDTA Grey top = fluoride oxalate Blue top = citrate
What is a purple-top tube used for?
Keeping cells alive.
Due to presence of potassium, can’t be used to measure electrolytes.
What is a grey-top tube used for?
Glucose testing. The fluoride oxalate is a poison which kills the erythrocytes, stopping the red cells performing glycolysis and preserving the glucose concentration in the sample.
What is the blue-top tube used for?
Measuring clotting factors in the blood
What is the difference between serum and plasma, and how would you produce serum?
Serum has no clotting factors in it: plasma has clotting factors.
To obtain serum, place sample in yellow or red-top tube (which don’t have any anticoagulant). The resulting clot can be removed to leave serum.
What is ESR?
Erythrocyte sedimentation rate - a measure of how quickly erythrocytes settle at the bottom of a test tube. A faster than normal (higher) ESR may indicate inflammation.
What are yellow/ red topped tubed used for?
Urea and electrolytes (U&E), thyroid function tests (TFTs), liver function tests (LFTs).
How do you obtain red cells and leave plasma?
Use a purple top tube - the anticoagulant (EDTA) prevents the clotting factors from being used. Erythrocytes will settle and can be separated from the plasma.
What proteins are measured in liver function tests?
Excess liver proteins in blood may indicate liver damage.
Enzymes measured: alkaline phosphatase (ALP), alanine amino-transferase (ALT), aspartate amino-transferase (AST), gamma glutamyl transferase (GGT).
Other proteins: albumin, bilirubin.
On an U&E, what does “HAEMOLYSED” next to the K+ measurement mean?
In poorly performed venepuncture, K+ can be released due to haemolysis, since erythrocytes contain lots of K+/ Instead of a yellow layer of plasma after centrifuge, you get a rose layer (free Hb).
Which markers rise in renal failure?
Urea and creatinine.
Which cardiac enzymes and proteins leak into the blood in cardiac damage, e.g. heart attack.
Troponins, creatine kinase (BM isoform), aspartate aminotransferase (AST) - also a liver enzyme - and lactate dehydrogenase (LDH).
In diagnostic virology, what types of things can we look for?
The infectious virus itself - virus isolation and electron microscopy.
Protein components of the virus (antigens).
Genetic components of the virus (DNA/RNA).
Host response (e.g. antibody).
Remind yourself of what the difference between a sensitive and specific test is.
Sensitive test: able to correctly identify positive samples.
Specific test: able to correctly identify negative samples.
What is “viral load”.
Quantification of the viral infection by determining the amount of viral genome in the blood.
Describe PCR briefly.
Use reverse transcriptase to form cDNA from RNA.
Denature at 95 degrees.
Cool to 50-60 to allow primer annealing.
Chain elongation at 72 using taq polymerase.
What is the main test in virology?
Serology testing (diagnostic identification of antibodies in serum).
What is the main test for detection of enteroviruses?
PCR - use a stool sample.
Vomit if necessary, but has a lower yield.
What are the common bacteriology diagnostic techniques?
Culture, serology, molecular techniques, susceptibility testing.
Susceptibility testing is the main reason we persist with culture.
What is a blood culture bottle?
A bottle which a sample of blood can be put in whose bottom becomes YELLOW is positive - flagging infection up to the investigator.
Describe what the gram stain test differentiates between.
Cell wall: gram positive has a thick peptidoglycan cell wall and only 1 membrane. Stains purple.
Gram negative has a thin wall and an outer lipopolysaccharide membrane (2 membranes). Stains pink.
What is the coagulase test used for?
Differentiating between staphylococci.
Coagulase is an enzyme produced by certain strains of staphylococci which converts fibrinogen to fibrin.
Staphylococcus aureus (including MRSA) test POSITIVE.
Commensal staphylococci on skin test negative.
A positive tests is visible as clumps.
What is the problem with testing stools?
A stool is a non-sterile site.
Unlike sterile sites such as blood and CSF, non-sterile sites physiologically have bacteria, making testing difficult.
Which bacteria are routinely looked for in stool samples? How?
Salmonella, shigella, campylobacter and E. Coli.
Different bacteria have different culture requirements: try to kill all other bacteria and only grow the one you’re looking for.
