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23 Cards in this Set
- Front
- Back
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Goals of testing
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Confirm suspicion of allergy, identify offending allergens, determine sensitivity.
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Why skin
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Much easier than intranasal challenge, lots of surface area, get an indirect assay of preformed IgE for that patient.
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What happens with skin testing
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When ag crosses barrier, picked up by APC, which migrates to LN, encounters a t cell with same specificity, which dances with b cell with same specificity, which then makes IgE which then take up residence on a mast cell in the skin.
On additional exposure, ag cross links IgE on mast cell, which degranulates and starts the allergic cascade. Amt of wheeling is proportional to the amt of IgE. Response peaks in 10-20 minutes. There is a biphasic peak of histamine, early and again rising from 6-12 hrs |
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What is the late phase response.
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Mutiple cell types, delayed inflammatory response. Local swelling and erythema. Not well studied. Not sure if a late phase only will benefit from IT treatment.
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Screening battery
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2 trees, grasses, weeds, molds, arthropods, and cat
95% certainty that you do not need further testing. |
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Patch testing
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Ag applied to skin with occlusive dressing placed on top. For contact hypersensitivity.
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Prick and puncture testing
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Picking starting dose for IT is trickey without IDT's
Controls Histamine a 1mg/ml conc. Saline and 50% glycerin. Less likely to get a false positive with SPT A positive prick corresponds to a #3-4 dilution IDT Goal with IDT is 4mm wheel. Cal be used to confirm a negative SPT. |
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IDT
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4 mm wheal will normally grow to 5mm by fluid dispersion.
7mm is necessary for a positive. Unlike SPT, you dilute the IDT . Done in 5 fold dilutions. Pro's quick, sensitive, reproducible. Cons's if test with too weak solution, may get false neg. if test with a strong solution, may get false positive. Most use #2-3 solution. Can skip dilutions when testing, go from #6 to #4 if first is negative. Consecutive dilutions helps us pick a safe starting pt for immunotherapy. |
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End point titration
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Progressive increase in strength of dilution yields progressively larger wheels.
Negative wheel < 7mm Positive is >\= 7 mm End point when wheal grows by 2mm with an increase in dilution. Confirming wheal when wheal grows again with an increase in dilution |
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Plateau
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When the wheel size stops increase with progressive strengths
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Flash response
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When get a huge jump in wheal size with progressive strengths. Need to stop testing here.
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How far do you titrate?
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Goal is to get to the #2 dilution.
Stronger might give you a response due to glycerine concentration |
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Diluents and preservatives
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Phenolated normal saline - inexpensive. Phenol for stability.
Albumin prevents antigen from adsorbing to the glass walls. Worry about transmission of infectious dz Glycerin- bacteriostatic, but with dilutions and lower concentration of antigen doesn't work as well. Goal is 10% glycerine to keep dilutions for 3 months |
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Controls
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Protects your test results from false positives and false negatives.
Histamine wheal is o.oo4 g/ml...... Also separate wheals by 2 cm. 5% of atopic population has dermatographism. Beta blockers can enhance the skin response, increases risk of anaphylaxis and difficulty of treating anaphylaxis. |
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Glycerin sensitivity
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Get a whealing response that increases with concentration. Compare this to your responses for antigens.
Want to see a 3mm wheal size difference between glycerine response and antigen to be counted as positive. |
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Drugs that are ok
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Ranitidine, cimetidine. Systemic steroids, ...
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MQT
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Modified quantitative testing
Use SPT. If wheal is <3 mm, is negative. If wheal is 3-8 mm, put a number 5 IDT. And go from there to find end point. If >\= 9 mm, end point is a #6. Less costly than IDT. Still get an end point, but we're not sure how well this correlates to IDT. Some evidence that SPT correlates with nasal challenge better then IDT. |
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In vivo vs in vitro
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On the pt vs taking a blood sample
In vitro means 'in glass' In vitro for dermatographia, chronic skin disorders, cant come off anithistamines, children, on beta blockers, prefer blood draw. |
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In vitro is done by...
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Ab to IgE, attracted to Fc portion, made from rats or other lab animal. Make a sandwich or chain with the tagged ab to IgE.
Label can be radiation, fluoresce, or enzyme linked. Gamma counter, vs light, vs color change..... More expensive, time delay, not as specific. |
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In vitro, grading system. rAST
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Creatednoff the gamma counter.
If greater than 50 counts, are class 4. If 20-50 , class 3..... Class 2-4 are clearly allergic, but difficult to tell between between 1 and negative. |
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Modified RAST, and others
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Subdivisions of the classes and changes technique allowed for greater specificity without losing sensitivity.
ICAP is fluorescence. Hytec - enzyme linked. |
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S IgE scoring
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Class 0 or o/1 are neg
Class1-3 are positive Class 4-5 most objectively sensitive, need to start at a more dilute starting dose. Score does not always correlate with degree of symptoms, just like SPT. Class 1 correlates with IDT #2. Tx with #1 2 corr. IDT#3. Tx with # 2 Class 3 corr with IDT 4 Tx with #3 Class 4 corr with IDT 5. Tx with #4 |
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Take home point for in vitro...
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Because they don't always correlate well, need to include history and physical in starting your end point. Only use in patients who wont tolerate skin testing.
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