CS Exam 2 Master Deck

Define serology

Identify and quantify a SPECIFIC ANTIBODY in the blood (based on antibody–antigen binding)

Define ELISA

Enzyme-Linked ImmunoSorbent Assay — immunoassay using an ENZYME as a label; substrate added to make signal; HIGH sensitivity

Define antibody titer

Highest serum dilution that still gives a POSITIVE result; higher titer = more antibody; follows disease progression

IM is caused by which virus?

Epstein–Barr Virus (EBV), a DNA herpesvirus. Infects B cells; atypical T lymphs seen on smear

Heterophil antibodies (definition)

Antibodies vs poorly defined antigens; "weak," poor specificity

IM heterophil antibodies are absorbed by

BEEF RBC antigen (non-IM = guinea pig kidney / Forssman antigen)

Clinically significant IM heterophil titer

≥ 1:56

HIV screening test

Detects ANTIBODY by immunoassay (immunochromatographic)

HIV confirmatory test

WESTERN BLOT — detects HIV antigens / viral proteins

HIV window period

18–45 days after exposure; before it → false-negative screen

HIV target cell

CD4 helper T-lymphocytes (killed → immunodeficiency); CD4<200 predicts opportunistic infection

HBV: all Ag and Ab negative

Not infected, NO immunity

HBV: all Ag negative, anti-HBs positive only

Vaccinated / immunized

HBV: HBsAg+, anti-HBc IgM & IgG+, anti-HBs–

Acute hepatitis, symptomatic

HBV: HBsAg+, anti-HBc IgG+, IgM–, anti-HBs–

Chronic hepatitis (no immunity)

HBV: all Ag–, anti-HBc+, anti-HBs+

Recovery from acute Hep B

HBV: HBsAg+, HBeAg+, all IgG Ab neg

Early infection, asymptomatic

Post-infection complications of S. pyogenes

Rheumatic fever + acute glomerulonephritis

Treatment for S. pyogenes

Penicillin (Rapid Strep test detects Strep A antigen)

RA autoantibodies

RF (rheumatoid factor, vs IgG) and ACPA (anti-citrullinated protein); also anti-Carp

Screening test for autoimmune disease

Immunofluorescence Assay (IFA)

Genus/species/strain of E. coli O157:H7

Escherichia = genus, coli = species, O157:H7 = strain

Why is Gram-positive purple?

Thick peptidoglycan traps crystal violet

Why is Gram-negative pink/red?

Thin peptidoglycan; crystal violet washes out, takes safranin counterstain

Why does M. tuberculosis resist Gram stain?

High lipid/wax (mycolic acid) in cell wall

Stain for M. tuberculosis

Acid-fast (carbolfuchsin) — Ziehl-Neelsen (heat) or Kinyoun (detergent)

Stain for Cryptococcus in CSF

India Ink (negative stain — shows capsule)

Complications of empiric wide-spectrum Abx

C-diff colitis, fungal infection, coagulopathy (loss of vit-K flora), drug resistance

Define pathogen

Causes disease in a susceptible host; TRUE pathogen infects healthy immunocompetent hosts

Define opportunistic pathogen

Normal/environmental flora causing disease when flora imbalanced OR immune defense compromised

Define iatrogenic infection

Infection caused by medical treatment or a procedure

Define virulence

Relative ability of a microbe to cause disease (degree of pathogenicity); measured by infective dose

Low infective dose means

HIGH virulence (Shigella ~100). High dose = low virulence (Salmonella ~10,000)

How does Protein A resist phagocytosis?

S. aureus Protein A occupies the Fc segment of IgG → blocks opsonization

Exotoxin properties

Secreted PROTEIN, heat LABILE, enzyme activity, effect SPECIFIC; 2 subunits (binding+toxic)

Endotoxin properties

LPS of Gram-negative cell wall, heat STABLE, NO enzyme activity, SIMILAR effects (fever, hypotension, coagulation)

Define bacteremia

Viable bacteria present in the blood

Define septicemia

Bacteremia + clinical signs/symptoms of invasion & toxin production; high mortality

Correct blood culture collection

NEW venipuncture (not a line/port); 3 sites; 20 mL/site = 60 mL total; 10 mL/bottle (aerobic + anaerobic)

Wound culture specimen of choice

BIOPSY (aspirate > swab)

Most important treatment of local wound infection

Surgical wound care & drainage (systemic Abx is supplemental)

Define pseudomembranous colitis

C. difficile overgrowth after wide-spectrum Abx; toxin causes cytotoxicity to epithelial cells

Most common cause of gastroenteritis in US children

Rotavirus

Best test for C. difficile

EIA for toxin (~3 hours) — fast and specific

Signature test for H. pylori

Urea breath test (detects radioactive CO2 the bacteria produce via urease)

Bacterial meningitis CSF findings

↑ protein (>100), ↓ glucose (<40), pleocytosis, predominant neutrophils

Signs of increased ICP

Headache, nausea/vomiting, papilledema

Meningeal irritation signs

Nuchal rigidity (stiff neck), photophobia, Brudzinski's sign

Do you wait for micro results before treating meningitis?

NO — start empiric wide-spectrum Abx immediately after obtaining samples

Define meningitis

Infection within the subarachnoid space (non-focal neural signs)

Define encephalitis

Inflammation limited to brain substance (focal neural signs)

Define meningoencephalitis

Infection of BOTH subarachnoid space and brain substance

Syphilis non-treponemal (screening) tests

RPR and VDRL

Syphilis treponemal (confirmatory) tests

FTA-ABS, TP-PA, MHA-TP

Preferred urine collection method

Clean-catch midstream

Positive pure urine culture colony count (per lecture)

≥ 75,000 CFU/mL → do ID & susceptibility (30k–75k case-by-case; <30k = probable contamination)

Next steps after positive culture

Full ID (genus/species) + susceptibility testing (Kirby-Bauer, MIC, E-strips)

Meningeal irritation signs indicate which two conditions?

MENINGITIS or SUBARACHNOID HEMORRHAGE (SAH) — irritation of the meninges, not necessarily infection

Define aseptic meningitis

Inflammation + pleocytosis in CSF with NO organisms observed on direct exam

Meningitis neural signs

NON-FOCAL neural signs

Encephalitis neural signs

FOCAL neural signs

Meningoencephalitis neural signs

BOTH focal and non-focal

Why is timing critical in suspected meningitis?

DO NOT DELAY. Untreated = poor prognosis: ↑ICP → ↓cerebral perfusion → brain cell death/herniation

Antibiotic approach in suspected meningitis

Start EMPIRIC WIDE-SPECTRUM Abx immediately after obtaining samples; narrow later per ID & susceptibility

CSF in bacterial meningitis (directional)

Cell count UP (pleocytosis), NEUTROPHILS predominant, GLUCOSE DOWN, PROTEIN UP

When are syphilis screening (non-treponemal) tests used?

FIRST — RPR/VDRL screen; detect Ab to substances from treponema-damaged cells (non-specific)

When are syphilis treponemal tests used?

AFTER a positive screen — FTA-ABS, TP-PA, MHA-TP CONFIRM; detect Ab against treponema itself

Direct visualization of Treponema pallidum

Darkfield microscopy (spirochete)

VRE — what does it mean and why does it matter?

Vancomycin-Resistant Enterococcus — resistant even to vancomycin; the WORST Gram-positive urinary pathogen

Staphylococcus saprophyticus — clinical association

Common cause of UTI in YOUNG WOMEN

Staphylococcus epidermidis — clinical association

Normal SKIN flora; common blood culture contaminant / device infections

Urine culture: ≥ 75,000 CFU/mL pure culture

POSITIVE → do ID & susceptibility on all specimen types

Urine culture: 30,000–75,000 CFU/mL pure culture

Gray zone — ID & susc. OR report "probable contamination"; departmental policy, case-by-case

Urine culture: < 30,000 CFU/mL pure culture

Usually report as "PROBABLE CONTAMINATION"

Urine culture: mixed growth

Usually "probable contamination" (case-by-case; if high count with one predominant organism, may ID that one only)

What does "too young — culture in progress" mean?

Preliminary report at 24 h: growth present but not yet enough to identify/report

Preliminary urine culture report at 24 h includes

"No growth after 24 h" OR colony count (CFU/mL) + probable contamination OR preliminary classification (Gram-pos / Gram-neg / mixed) + "susc. & IDTF"

How long do you hold cultures for fungi?

4 WEEKS (filamentous fungi are slow-growing)

How long do you hold cultures for mycobacteria?

6 WEEKS

When are negative bacterial cultures reported?

After 24–48 hours of incubation (per lab policy)

Kirby-Bauer disc diffusion — how to read it

Measure the ZONE OF INHIBITION around the disc: LARGE zone = SENSITIVE, small/no zone = RESISTANT

MIC interpretation

LOW MIC = SENSITIVE (little drug needed to inhibit). HIGH MIC = RESISTANT

Standardized susceptibility methods

Kirby-Bauer (disc diffusion), MIC testing, E-strips

Most common CAP pathogens

Streptococcus pneumoniae, Haemophilus influenzae; atypical/walking = Chlamydia & Mycoplasma pneumoniae

Most common cause of pneumonia AND meningitis in a NEONATE

Group B strep = Streptococcus agalactiae

Common Gram-positive urinary pathogens

Enterococci (worst = VRE), S. agalactiae, S. pyogenes, S. saprophyticus, S. aureus, S. epidermidis

Common Gram-negative urinary pathogens

Enterobacteriaceae (E. coli, Proteus, Klebsiella, Serratia, Enterobacter, Providencia, Morganella) and Pseudomonas aeruginosa

Common fungal urinary pathogens

Candida albicans, Candida glabrata (Torulopsis glabrata)

CSF tube order

1st = chemistry, 2nd = microbiology, 3rd = heme (cell count & diff), 4th = serology/special

CSF specimen handling

Transport ASAP; DO NOT REFRIGERATE (room temp); centrifuge — culture & Gram stain from sediment; BAP, CHOC, Thio

Fungal meningitis in immunocompromised

Cryptococcus neoformans (also histoplasma, candida) — India Ink on CSF