Basic Lab in Infection

What is treponema pallidum

Spirochete that cause syphilis; cannot be seen with gram stain, has cell wall

Syphilis labs

Non treponemal: VDRL, RPR

Treponemal: TPHA, FTA-ABS, Immunoassay, Dark field, PCR

What do VDLR and RPR measure

Look for agglutination reaction between Ag suspension and Px serum with Ab

On RPR and VLDR what do higher titers agglutination indicate

Higher titer = more diluted → If still positive after a lot dilution = high amt of Ab

What is TPHA test

Use heme to test for syphilis Ab → Ag to RBC to test for Ab present → If yes there is agglutination

Labs for Rickettsia

IFA (IgM, IgG), ELISA, Rapid test, PCR

Typhoid and paratyphoid fever labs

Culture (gold standard), Widal test

Typhoid and paratyphoid fever culture can be sent when

Blood culture in wk 1

Stool/urine in wk 2-3

Widal test has what Ag

– O Antigen on S typhi → Current/active infection

– H antigen of S. typhi: Past or recent infection,

vaccination

– AH antigen of S. paratyphi A: Infection with Paratyphi A

– BH antigen of S. paratyphi B: Infection with Paratyphi B

O Ag positive with AH Ag positive means what

Current paratyphi type A infection

What is paired sera for Widal test

Take two blood samples measured 7-14 days apart, look for >=4 fold increase in Ab titer

Ab kinetics of Typhoid and paratyphoid fever

Wk 1: Ab not detectable, blood culture +/ve in 90% of case

Wk 2: Anti O appears first (d6-8) then Anti H (d10-12)

Wk 3-4: Both reach high level → Do Widal

Wk 5+: Anti O rapid decrease, Anti H constant elevation

Why is Anti O good at indicating re infection but Anti H is not

Anti O decreases after wk 3-4, if it suddenly increase it mean re-infection

Anti H remains elevated for a long time

TB labs

AFB, NAT, mycobacterial culture, drug susceptibility testing

H. pylori test

Urea breath, stool Ag, serology, gastric biopsy

What is the drawback of serology test in H. pylori

Cannot tell us if the infection is past or current

Gold standard for H. pylori lab

Stool Ag → Indicate ACTIVE infection

HIV test

Strip test at 14-20 and 21-30 days

NAT at 3-7 or 7-14 days → Used for blood donation

Viral load

CD4/CD8

Drug resistance

What is the difference between 3rd and 4th gen HIV strip test

3rd gen has Ab only → Positive 21-30 days after infection

4th gen has Ab and p24 → Positive 14-21 days after infection

HBV labs

HbsAg (must), Anti-Hbs (must), Anti-HbC, Anti-Hbe, HBV viral load

How do we know that a vaccinated person has protective effect against HBV

Anti-Hbs must be >=10 ml/U in order for it to be protective

Dengue labs

NS1 Ag, IgM, IgG, ELISA, RT-PCR

Do labs diff the DENV subtypes

No, the only test that can diff is RT-PCR

What does NS1, IgM and IgG tell us

NS1 detectable at day 1-5 → If negative after d5 does not rule out

IgM detected day 4-5 of fever, peak at 2 week and persist for 2-3 month

IgG detected day 7 in primary, rapidly in secondary

HPV labs

Molecular = HPV DNA (16 and 18), mRNA (E6/E7), urine; Pap, liquid based; Rapid

What is the difference between malaria thin and thick films

Thick → Confirms if there is ANY malaria

Thin → Tells us what type of malaria it is

What is HRP2, pLDH, and pV-pLDH

HRP2 → Specific in FALCIPARUM only

pLDH (pan) → Positive for all malaria

Pv → Positive in VIVAX only

What strain of malaria is this

Pan, Pf positive, Pv neg → FALCIPARUM

What strain of malaria is this

Pan and Pv positive → VIVAX

What strain of malaria is this

Only pan positive → Non falciparum, non viva

What strain of malaria is this

All positive → Mixed infection