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Lab diagnosis

Lectures 19-20

non-specific signs/test physical: fever, pain, swelling, redness general lab tests: CBC (increased WBC), non-specific (increased RBC sedimentation rate, elevated CRP), localized changes (increased protein in CSF, abnormal liver function)
accuracy vs. precision precision: how consistent are the values accuracy: how close are the test values to the true value
predictive value a measure of the times that the value is the true value
impact of incidence on sensitivity a higher prevalence leads to better performance in a test
immunodiagnosis of acute infections draw an acute serum (before 14 days), draw a convalescent serum 3-6 weeks later (look for a 4 foldor greater increase due to IgG). Most use ELISA now so 4 fold increase in optical density
If you miss the acute titer... draw a convalescent titer at the peak (3-6 weeks after start of infection) A very high convalescent titer=infection presumptively confirmed. A single positive Ab does not allow definitive confirmation (exception: lyme diseae, HIV, hepatitis)
diagnosis of chronic infections HIV/lyme disease: western blot Hepatitis B: several Ab markers HIV/Hepatitis B and C: can also check for the presence of the Ag by PCR and Ag immunoassay for Hepatitis B
Test for congenital infection check Ab titer against agents suspected, test cord blood serum at birth, test again after 3-4 months. Baby negative: IgM not present, IgG from mom will disappear at 50% drop rate every month Baby positive: IgM positive and IgG will go up
Third generation immunoassays sensitivity and specificity are typical of enzyme immunoassays single analyte tests: easily detect one thing, sold phase or membrane based, convenient to perform
Fourth generation assay detect two things or use novel technology simultaneous: Ag and Ab or multiple Ag kites made with superior reagents: monoclonal Ab or recombinant Ag New improved detection technology: time resolve fluorescence, immunoprobe for fiber optics
First molecular nucleic acid amplification method target amplification: PCR target fingerprint-->amplification and probe capture-->detection
emerging diagnositc technologies microarrays: for microbial identification and typing, detection of host responses to infection. Unique to microarrays (Can do 10 to 20000 tests on a simgle specimen)
types of arrays microbe oriented-look for pathogen host oriented-check for the patient's response to infection, check for host characteristics that predict worse outcomes or problems with a particular drug
technology summary cultures give positive vs negative answers chemistry styles give grey answers: color production with varying degrees of intensity, can vary from test to test important that patients realize the possibility of false pos/neg
disease prevalence affects predictive values increasing prevalence increases the PVP/PVN
situations for rejection of specimens that are beyond salvage misidentification, culture specimens received in fixate, dried out specimens on swab, insufficient quality
criteria for rejection of seriously compromised specimens improper storage temperature, improper transport medium, improper transport time, leaky specimen
organisms requiring prompt culture if specified by doctor Haemophilus ducreyi, anaerobes in regular transport tubes, Neisseria gonorrhoeae in joint fluid
contaminated specimens expected: contain normal flora that needs to be distinguished from pathogens unexpected: sterile with introduced bacteria
blood cultures continuous bacteremia: volume dependent, collect 20-30 ml per set, 2-3 sets per episode intermittent bacteremia: 2-3 sets before antibiotics collect each set from a new venopuncture site: two sticks is a minimum
things to avoid in blood culture collection disinfecting collection site with alcohol only(iodophor for 2 minutes),initially collecting >5 blood culture sets to diagnose a single clinical episode,collecting only one bottle per set(1 aerobic and 1 anaerobic bottle),collecting all from 1 needle stick
blood culture issues 3% increase in yield/mL (collect 20mL/set and 2-3 sets), central venous catheters (semiquantative culture of CVC tip: >15 significant), postmortem blood rarely useful
true positives vs. contaminants true positives: multiple sets and bottles are positive, positives arise within 3 days, organism isolated is typical pathogenContaminant: only one bottle positive, organism normal skin type flora, takes more than 5 days to grow
CSF wear mask to avoid normal oral flora in tubes, do gram stain drom cell count tube to protect culture tube (should see organism in gram stain 70% of time) contaminant have negative gram stain and <3 colonies air contaminants are not on streak line
detecting agents of pneumonia best sample is detection of lung tissue collected in OR, second best is saliva free sputum collected by bronchoscopy, third best is good cough sputum
cough sputum cultures avoid saliva (small quantities of thin clear liquid is from mouth...will have squamous epithelial cells and few WBC)
urine cultures bacterial growth: doubles every 30 minutes pathogen and vaginal flora may mingle (refrigerate to preserve true numbers)
quantitation and significance of bateruria likely to be significant 80% of time (100000 CFU/ml of one organism) in special circumstances 10^2 or 10^3 CFU/mL may be significant in men or symptomatic women
urine collection clean catch, first morning specimen refrigerate or put in special preservation tube
urine culture preservation refrigeration alternative, boric acid based, mildly toxic, dilution onto media initiates growth. Colony counts stable for 8-24 hours at 20C
quantitation and significance of bacteruria contaminated specimens (high quantities of >3 organisms), correlation with significant bacteriuria: gram stain (at least 2 bacteria per high power field has 90% correlation and significance)
Unsuitable specimens for lab detection of UTI's foley catheter tips, urine from catheter bag, unsupervised clean catch specimens from female patients who have not had instructions, urine over 2 hours old at 20C
community acquired diarrhea occurs before 3 days in hospital, may be due to parasites, enteric bacterial pathogens, viruses
Hospital patients with diarrhea C. difficile, in hospital over 3 days, on broad spectrum antibiotics or chemotherapy
wound specimens tell lab the anatomical site, surface wounds have skin flora as pathogens and contamination, don't sample superficial pus, deep wound sample okay
Do's and don'ts for likely contaminated surface wounds don't swab superficial layers or put swab through contaminated superficial layer trying to go deeper, don't use surgical techniques to take a superficial specimen with surface contamination
rejections related to anaerobes don't culture from these anaerobes: fecal contaminated specimens, midstream and catheter urine, expectorated sputum, throat nose orophayngeal swabs, gastric contents, vaginal and cervial swabs, superficial material from skin
Minimum inhibitory concentration least antibiotic concentration that inhibits growth
problems with MIC tests can be expensive, each drug takes multiple tubes
disk diffusion test mueller hinton agar plate seeded with organism, position paper disks impregnanted with antibody, measure zone in mm
problems with disk diffusion overnight incubation required, average accuracy 92%, increasing number of drug/bug specific contraindications limits ability to be a universal test
E-test Antibiotic is impregnanted onto a rectangular plastic strip, like disk diffusion, but strip is placed on the agar
Extended spectrum beta lactamase plasmid mediating beta lactamases that confer resistance to penicillin, cephalosporins, and aztreonam. Mainly in klebsiella and E.Coli
ESBL enterobacteriaceae appear susceptible in vitro to second generation cephalosporins and resistant to first and many of the third generation
inducible beta lactamases enzyme conferring resistance to penicillins and cephalosporins in certain gram negative rods. Delayed and develops slowly. May occur with klebsiella, citrobacter, enterobacter, E.coli, aeromonas, pseudomonas, and some others
Inducible AmpC beta lactamase just became ready for limited routine testing, on plasmid in some enterobacteriaceae
Klebsiella Pneumoniae Carbapenemase beta lactamase, confers resistance to all beta lactams, occur in enterobacteriaceae, most commonly klebsiella. Rarely in pseudomonas
lab detection of KPC producers problem: some isolated demonstrate low level carbapenem resistance, some automated systems fail too detect low level resistance
When to suspect a KPC producer enterobacteriaceae (esp. Klebsiella) that are resistant to extended spectrum cephalosporins, and resistant or intermediate to cefoxitin and cefepime
empirical antimicrobial therapy is based on historical susceptibility patterns, body site and acuity of disease, likely organis, involved, clinical trial data
Created by: kamarsh



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