Blood Testing

Blood Testing in Lab

The Scott & White Blood Center is registered by the FDA and accredited by AABB, CAP and the Joint Commission. Our laboratories meet the requirements of the CLIA. Blood collected at our facility is screened for all required infectious disease markers by nucleic acid amplification testing methodology.

There are eight tests that must be performed on all donated blood, as required by Food and Drug Administration (FDA) regulations and guidelines. At Scott & White, these tests are performed on all units donated for general use (allogeneic blood). The tests are not required on autologous units.

The tests are as follows:

  • MHA-TP - microhemagglutination assay-treponema pallidum, - a specific serologic test for syphilis
  • HBsAg - test for the hepatitis B surface antigen
  • Anti-HBc- antibody to the core antigen of hepatitis B
  • Anti-HIV 1, 2 - antibody to the human immunodeficiency viruses 1 and 2
  • HIV and HCV by NAT - Test for the genetic material of these two viruses using Nucleic Acid Techniques
  • WNV by NAT - Nucleic acid technique assay for West Nile Virus

All of these tests are performed in a FDA-approved reference laboratory (BSL, Bedford, TX ). When donors meet the regulatory criteria for deferral they are sent a letter regarding the positive test. Not all letters name the test because of concerns about privacy. Letters about HTLV, HIV, and HBsAg do not name the test, so please call the Medical Director or make an appointment to get the specific results. These results are maintained in a confidential manner. If the donor is a Scott & White patient, the results are not sent to the patient's medical record.


This is a test for antibodies to the organism that causes syphilis. Unfortunately, it is often falsely positive, which means it is reactive but the person does not have syphilis. We determine this in the laboratory by performing a confirmatory test called the IgG- enzyme immunoassay (EIA). When the MHA-TP is reactive we automatically perform an IgG- EIA. If the EIA is negative, the MHA-TP is considered a false positive. We recommend that anyone who has a reactive EIA see their doctor to make sure they get treatment if they are infected.

Blood that is MHA-TP reactive cannot be used for transfusion to patients (the FDA does not distinguish between true and false positives). We defer all MHA-TP positive donor for one year. We recommend retesting after 12 months to see if the MHA-TP has become negative. This retesting is available in the donor room and requires only a small sample of blood. Unfortunately, the MHA-TP usually remains positive for many years.

Some donors who had a positive MHA-TP will return to the donor room with a letter from their physician indicating that there is no evidence of syphilis, or that it has been successfully treated. This is acceptable, but we must test a sample to assure that the MHA-TP is negative. Only when the MHA-TP becomes negative can the blood be used for transfusion.

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Test Tube RackThis is a very useful test for the hepatitis B virus, a major cause of liver infection that is spread by blood contact. A confirmed positive test defines the carrier state for this virus. The initial result is confirmed by a neutralization procedure. Everyone infected with hepatitis B develops HBsAg, but most recover and clear the virus. They then may lose the HBsAg over time. Some people, however, become chronic carriers of the virus and remain HBsAg positive. These people can transmit the virus through their blood and that is why the test is used. Those positive for HBsAg should see their doctor for evaluation of their liver function and possible treatment. Anyone positive for HBsAg is permanently deferred as a blood donor.

It’s uncommon, but sometimes the HBsAg is not confirmed by the neutralization procedure. This is a false positive pattern and the donor is not deferred. They are placed on surveillance and allowed to continue donating. A donor is deferred only when the HBsAg is positive on a second occasion (whether it is confirmed by neutralization or not). The deferral is permanent.

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This tests for the antibody to the core antigen of hepatitis B and, apparently, a number of other antigens similar to it. It is a useful test for confirming previous exposure to hepatitis B in patients. When applied to a healthy blood donor population, however, it is a very non-specific test. This test is the most common cause of donor deferral, over a hundred a year at Scott & White. We have followed many of these people for years and they do not develop hepatitis or any other disease. Possibly as many as a third have had hepatitis B, recovered completely and are not carriers . They have anti-HBc from that infection and also will have anti-HBs, the protective antibody. The remaining two thirds have no evidence of hepatitis and their positive test is probably the result of some other stimulus to their immune system (other viruses, flu shot, etc.). Persons who have received the series of hepatitis B vaccine shots will have anti-HBs, not anti-HBc.

The real problem with anti-HBc testing is that no confirmatory test is available to tell us which are the true positives. This is the only required test that has no confirmatory test. Experience has taught us that almost all are false positives, but we cannot prove it. The FDA recognizes this lack of specificity and does not require donor deferral until the test has been positive on two separate occasions. We do not transfuse anti-HBc positive units, but we do place the donor on surveillance. There is a very good chance that their next donation will test negative and that unit can be transfused. Only when the test is positive a second time do we notify and defer the donor.

We advise those who have a positive anti-HBc,and are feeling well, that there is no cause for alarm. They may want to keep the letter and show it to their doctor at their next appointment, but it is not a reason to make an urgent appointment. Anyone with a positive test who is not feeling well should seek medical evaluation. Because there is no confirmatory test, re-entry as a blood donor is not possible.

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Anti-HTLV I / II

Test TubeThe human T-cell lymphotropic viruses are rare in this country and positive tests are rarely confirmed (i.e., almost all are false positives). A word of warning to those looking up “HTLV” on the Internet: You will find many references to the AIDS virus, HIV, because it was originally called HTLV-III. We are not talking about the AIDS virus here. HTLV-I is endemic to Japan and has been associated with a rare form of leukemia and a nerve disorder that causes spasticity. These diseases occur in infected individuals in 3–4 percent of cases, often 10–20 years after infection. So, even in people truly infected, the risk of disease is low. When the initial screening test is positive, the sample is tested with a second screening test, made by a different company. If the second test is negative, the donor is placed on surveillance status but is not notified. The unit is not used. If the second test is reactive, a Western Blot confirmatory test is done and the donor is notified. In most cases the confirmatory Western Blot is negative or 'indeterminant'. Either of these results indicates a false positive screening test. As with all of our tests, additional follow-up testing is available upon request. As with the RPR and anti-HBc tests, you are not deferred until the test is positive on two occasions, unless the Western Blot is positive.

No disease has been associated with HTLV-II.

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Anti-HIV 1, 2

This is the screening test for antibodies to the virus that causes AIDS. The confirmatory test is the Western Blot, which may be positive, negative or indeterminant. We also now have available the NAT results to help evaluate true versus false positive. Confirmed positives are advised to seek further evaluation and treatment through their doctor. Most physicians perform a quantitative HIV test by molecular methods (usually PCR, polymerase chain reaction) for final diagnosis and treatment planning. In most cases, however, the Western Blot is negative or indeterminant, a false positive pattern. NAT is usually negative, also confirming a false positive screening test. We recommend re-testing for unconfirmed positives to see what happens over time. In some cases the test becomes negative, confirming that it was a false positive. In very rare cases the indeterminant Western Blot becomes positive, indicating that the person was recently infected and was just beginning to make antibodies when initially tested. The NAT should also be positive in these cases. If the results are unchanged on re-testing, this confirms that it is falsely positive, since someone truly infected would have converted to a positive Western Blot or NAT within a few weeks. All donors with a positive screening test are deferred permanently and are not eligible for re-entry.

Despite all the scientific reassurance, it is deeply unsettling for most people to be told they have a positive AIDS test, even a falsely positive one. From the donor’s point of view, the key question is whether there has been a possible exposure. If there is no possibility of contact with the virus, then the false positive test is just a laboratory anomaly. If the possibility exists of a high risk exposure, then you should definitely repeat the test and consider seeing your doctor. Remember that all test results from donated blood are kept in strictest confidence and are not in your medical record unless you request they be put there. Many insurance companies are testing for HIV and even a false positive test may cause them to deny coverage, so discuss with your doctor what should be written in the record.

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Biohazard SymbolThis is the screening test for antibodies to the hepatitis C virus. Positives are confirmed by recombinant immunoblot assay (RIBA), which may turn out positive, negative or indeterminant, and NAT testing for HCV. Confirmed positives are referred for evaluation to their physician. Negative or indeterminant RIBA’s indicate a false positive screening test, which can be further confirmed by negative NAT results.

Chronic carriers of hepatitis C are at increased risk of cirrhosis, or scarring of the liver. Those who develop cirrhosis are at increased risk of liver cancer. Treatment is available, so it is important to have periodic checkups.

Donors with a positive anti-HCV are permanently deferred, regardless of confirmatory testing results and are not eligible for re-entry at this time.

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HIV and HCV by NAT

Scott & White Blood Center is also using molecular methods to test for the presence of the genetic material (RNA) of Hepatitis C and HIV. This uses a nucleic acid technique (NAT) and is very sensitive and specific. The increased sensitivity of NAT closes the "window" between exposure and a positive test to 11 days for HIV and 23 days for HCV, meaning that infectious units will be picked up much sooner. This test is done by Blood Services Laboratories of Bedford, Texas on pools of 16 donors.

NAT results will also help clarify false positive HIV and HCV results. If there is a positive screening test, but negative or indeterminant confirmatory test, a negative NAT result will establish that it is a false positive screening test. Although rare, a positive NAT will confirm that the individual is infected.

So far, NAT is only for HIV and HCV, but it can theoretically be expanded to cover all diseases transmitted by blood transfusion. Someday we may be able to drop the tests that cause so many false positive reactions (especially the MHA-TP and anti-HBc) and re-evaluate those who had false positives. It may be possible to reinstate those who are NAT negative as eligible blood donors. If you had a false positive test, we keep that information on file and will notify you when improved testing allows re-entry.

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Beginning on July 1, 2003, all donated blood has been tested for West Nile Virus by NAT. Since this test is not yet licensed by the FDA, this testing is being done under an Investigational New Drug protocol (IND). This is why there is a separate fact sheet on WNV and a special informed consent. Anyone testing positive on this test will be asked to give additional blood samples over the next few weeks to see when the NAT becomes negative and the antibody to WNV appears. At this point the person can be re-entered as a donor, since they will have cleared the virus and become immune.

WNV is usually spread by mosquito bite, but can be transmitted by blood transfusion or organ transplantation. Infected persons are often asymptomatic. Some have a flu-like illness with fever and headache. Mortality rate is about 5 percent, with the very old and very young especially at risk.

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