LABS; 1, 2, 3, 4 and 5 - Diagnostics - Intro and chemical pathology labs, Virology lab, Bacteriology lab, Histopathology and cytopathology lab, Antibodies as diagnostic tools/Case summary Flashcards
How can you diagnose a virus?
History, physical examination and lap tests (non-specific and virological)
What can you detect in the lab?
Infectious virus (virus isolation and EM), Protein components (antigens) of the virus, Genetic components of the virus -> quant/qual tests available; host response (antibody/cell responses)
What are the diagnostic methods used in the lab?
Cell culture, electron microscopy, Antibody detection (serology - EIA), antigen detection (Immunofluorescence, enzyme immunoassay - EIA), genome detection (PCR), quantification of Ab/Ag, serotyping, quantification of genomes (viral load - essential for diagnosis and monitoring of HIV, HBV, HCV, and also for CMV and EBV in immunocompromised), genome sequencing (genotyping, antiviral resistance testing)
What are the limitations of lab tests?
All assays give rise to false negative and false positive results -> sensitivity (tests ability to correctly identify positive samples); specificity (tests ability to correctly identify negative samples)
Where are the typical samples taken?
Throat swab, Nasopharyngeal aspirate, bronchoalveolar lavage, ET secetion for resp viruses by PCR; Stools for rotavirus, adenovirus and norovirus Ag detection or PCR; Urine for BK virus and adenovirus by PCR; CSF for herpes viruses and enteroviruses by PCR; Blood (clotted) for serology Ab detection; Blood (EDTA) for PCR/viral load testing; saliva for serology and PCR (measles)
What serology is looked for?
How is IgM different from IgG?
How do you carry out Ab avidity testing?
What tests do the chemical pathology lab carry out?
LFT, Urea and Electrolytes, blood glucose
When collecting blood what should you do?
Use the correct tube; label the tube with patients details
What is inside the tubes with coloured lids - red, yellow, purple, grey?
Red - nothing; Yellow - gel to speed up clotting; Purple - potassium EDTA to prevent blood clotting and preserve cells Grey - fluoride oxalate (poison) which kills RBC so they don’t take up the glucose in the blood
What would you collect in each type of tube?
U+E serum in Y/R; glucose plasma in grey; HBA1c plasma in purple; TFT serum in Y/R; LFT in yellow/red
What is the difference between serum and plasma?
No anticoagulant, Blood clots using up clotting factors and then clot can be removed = serum; Anticoagulant, EDTA or heparin, clotting factors unused and blood separated into red cells and plasma = plasma
How do we test for clotting factors?
Citrate in blue tube chelates Calcium which stops the blood from clotting; time how long it takes to coagulate
How do you measure glucose?
Grey tube -> Fuoride oxalate prevents red cells from using glucose as it is a poison
What other components can be tested for in the blood?
Calcium and phosphate, markers of liver function (liver enzymes should be in the liver but if they leak then there is a problem, and some enzymes are specific to certain diseases - akaline phosphatase, AST, ALT, GGT), hormone assays, glucose
What causes low Na and high K?
Low Na is due to dehydration, K due to kidneys not functioning well
What is the meaning of Urea and Cr levels?
Both should be excreted by the kidney so measure renal function -> Cr should be excreted at a fixed rate; high urea is due to dehydration as it absorbs the water as well as the urea
What is the commonest cause of high K in the lab?
Haemolysed blood as RBC contain large amounts of K which leaks pout when haemolysed
What happens to urea and creatinine in renal failure?
Both rise but if creatinine is normal and urea rises then it is dehydration
What is creatinine?
Marker of glomerular filtration rate. If it is normal, then the GFR is normal. Very little is absorbed or secreted by the tubules.
What happens to urea levels when patient is dehydrated?
Levels rise but GFR stays the same to the end
What are the liver enzymes?
Present in liver and tiny amount leaks into blood -> liver disease more of these enzymes leak into the blood
Which liver function and enzymes are tested for?
AST, GGT, Albumin (synthesised in liver), bilirubin, alkaline phosphate, ALT
Why are cardiac enzymes are measured?
Present in heart muscle and during a heart attack, heart muscle is damaged, which causes enzymes to leak into the blood in large amounts -> then can check if heart attack actually happened
What cardiac enzymes are measured?
Troponins, creatine kinase, AST, lactate dehydrogenase
How would you test for HIV serology?
4th gen EIA (Ab and p24 Ag detection)’ all reactive samples undergo confirmatory testing in a second assay to excude non-specific reactivity (false positive), confirmed positives undergo typing HIV 1 vs 2; repeat blood sample +EDTA blood for HIV viral load required from all new +ves
When is virus isolation in cell culture used?
Rarely as it is slow, tme consuming, still useful for phenotypic antiviral susceptibility testing (HSV)
When is electron microscopy used?
Viruses too small to be seen by light microscopy, can be visualised using electron microscope -> sample types such as stools and vesicle fluids (rarely used)
When is immunofluorescence used?
Direct detection of viral antigens in clinical samples, used for typing and cell culture confirmation, rapid an inexpensive but subjective and very dependent on skill of technician and quality of sample