Methods, Laws, and Issues

(Updated 16 March 2007)

What to test for

• Blood banks
  • test for HIV1 and HIV2
  • set sensitivity very high (lots of false positives)
• Personal tests (screening)
  • usually only HIV1
  • usually only for antibody
  • not reported (+) unless confirmed

Kinds of HIV Tests

• Test for presence of antibodies
• Test for presence of HIV

Differences in the Tests

• What is detected?
  • antibody
  • antigen (protein from HIV)
  • DNA integrated in cells
  • RNA of free virus or active infection
• What binds to the target of detection?
• How is the signal amplified

Antibody tests: ELISA

• “Enzyme-linked immunosorbent assay”
• Detects presence of anti-HIV antibodies
• Start with antigen-coated beads or slides
• Add serum (blood) to be tested
• Beads get coated with antibodies or not
• Then presence of antibodies detected by color test

More detailed version

• Mix antigen-coated beads with serum to be tested.
• Wash beads to remove all antibodies that are not specific for the antigen on the beads
• Mix the beads with a “tagged” antibody from goats that binds to human antibodies
• If no human antibodies were able to bind the antigen coated beads, no goat antibody will bind
• If the antigen-coated beads were covered with anti-HIV specific human antibodies from the blood, then the goat antibodies will bind all over the place
• Goat antibody is linked to an enzyme that keeps converting a colorless compound to a very colorful one.
• If no “tagged” goat antibodies on the beads, no color develops
• If beads have lots of “tagged” goat antibodies, color keeps developing.

Results of ELISA Test

• Lots of false positives
  • the test is purposely made too sensitive
  • must retest to be sure using another test that is less sensitive, but more specific.
• Some false negatives
  • window period between active infection and first signs of an immune response against HIV

How are ELISA data used

• Blood banks
  • A single positive and the blood is sterilized and discarded
• Personal tests
  • The test is repeated twice more
    • if both are negative, result is reported as negative
    • if one or more is positive, do a Western Blot before reporting

Western Blot

• Also detects anti-HIV antibodies
• Great for showing specific vs. non-specific
• More expensive (ten-fold) and more skill required
• Still a screening test

How do Western Blots work?

• Separate HIV proteins by size and “blot” to special paper
• This establishes a very specific pattern or picture of where the proteins are AND WHERE THEY ARE NOT! m
• Add serum (blood) to be tested.
  • If anti-HIV antibodies present, they bind where proteins are and repeat the pattern
  • If non-specific stuff is there, it binds randomly or at least in the “wrong” places.
• Detect human antibodies with goat anti-human antibodies and a color reaction again.
• Advantages of Western Blots
  • Very specific
  • Done on same samples as ELISA
• Disadvantages of Western Blots
  • Not very sensitive (only useful if ELISA positive)
  • Not easy to “standardize”

Sensitivity and Specificity

• Knowing the precise frequency of false positives and false negatives from ELISA vs. Western Blot, can use a correction factor to get real prevalence of HIV (+) in the population from ELISA alone

Nucleic Acid based tests

• PCR to detect DNA (in cells)
• RT-PCR and branched DNA to detect RNA (in cells or in viruses)

PCR (Polymerase Chain Reaction) (See Cartoon in lecture)

RT-PCR

• Reverse Transcriptase + PCR
• Reverse transcriptase converts RNA into DNA (remember, that’s how retroviruses like HIV work!)
• Then PCR amplifies DNA as before

Sensitivity of RT-PCR

• Can detect a about 500 RNA molecules per ml of blood.
• VERY sensitive!

Names Reporting and Testing

• CDC does not keep names for HIV
  • County and state sometime do
• AIDS vs HIV
  • 50 states require AIDS reporting by name
  • 33 (including Michigan) require HIV to state (by name)
• AIDS reporting tracks infections of previous decade. HIV is closer to present

Some differences

• 12 states list HIV as an STD
• 16 states list HIV as a communicable disease
• 23 states list HIV as a “separate catagory”
• 12 states allow physicians to notify partners

Some interesting numbers

• About 30% of HIV(+) persons do not know that they are positive
• About 30% of those tested for HIV do so to find out their status
  • the rest are meeting some requirement
    • blood donation

Should HIV Testing be Compulsory?

The present status

• 29 states allow involuntary testing under some conditions
• 27 states require parental consent for testing of teens
• 23 states (Michigan included) allow teens to consent without parental permission
• otherwise testing is “battery” or “unlawful touching”

Voluntary Testing

• Names or no names?
• how to follow up with counselling if no names?
• Voluntary can give a skewed representation
• are highest risk persons most likely to be tested or least likely?

Mandatory Testing

• Mandatory: not forced. Required for some other non-required activity
  • give blood
  • travel overseas
• Syphilis testing was mandatory
  • not cost effective (only 1% of new cases discovered this way
• Problem is confidentiality

Compulsory testing

• 44 states (Michigan included) can compel HIV testing of those convicted of or charged with sex offenses.
  • NOTE: even if only charged!
• 10 states (Michigan included) compel testing of convicted prostitutes
  • continued unsafe sex is then a felony!

The Confidentiality issue

• This is central to the debate
• Loss of job, insurance, etc.
• Social stigma
• Homosexuality and IDU are still illegal in many states (Michigan included)
  • In Michigan, “sodomy” is a felony — but is not enforced in Wayne county.

Do Partners Disclose? — One study:

• 40% did NOT tell their partner
  • 58% of those did not use condoms regularly
  • mostly poor, IDU, and lacking a high school diploma
  • blacks were 3 times less likely to tell partners than other groups
• Why not tell?