Guidelines / Advice

What is NAT ?
NAT stands for Nucleic Acid Testing.

What are nucleic acids ?
Living organisms are complex systems. Many thousands of proteins exist inside each one of us to help carry out our daily functions. These proteins are assembled piece-bypiece to exact specifications. This information, detailing the specific structure of the proteins inside of our bodies, is stored in a set of molecules called nucleic acids.

Deoxyribonucleic acid (DNA) is a nucleic acid that contains the genetic instructions / information specifying the biological development of all cellular forms of life. DNA is made and resides in the nucleus of living cells. DNA gets its name from the sugar molecule contained in its backbone (deoxyribose), however it gets its significance from its unique structure. There are four different nucleotide bases that occur in DNA:
adenine (A), cytosine (C), guanine (G) and thymine (T).



In simpler terms, DNA is a basic building block of life and
contains all the information that living things need to function correctly.

Ribonucleic acid (RNA) gets its name from the sugar group in the molecule's backbone - ribose. Unlike the doublestranded DNA molecule, RNA is a single-stranded molecule, RNA can move around the cells of living organisms and thus serves as a sort of genetic messenger, relaying the information stored in the cell's DNA out from the nucleus to other parts of the cell where it is used to help make protiens.

The window period

After the onset of HIV infection the body requires a certain period of time to produce antibodies. The window period is the time from infection until the time that there is sufficient antigen or antibody in the blood to produce a positive test result.

Prior to the introduction of NAT the window period could be anything from 14 - 22 days. This meant that the HIV blood test can be negative during this period even though the person has been infected with HIV. If this donation is transfused the virus can be transmitted to the patient receiving the blood. Thus - it is of utmost importance to try and narrow down the window period as far as possible with laboratory tests.

How did we test for HIV before the introduction of NAT ?

Every blood donation is routinely tested for Syphilis, Hepatitis B & C and blood group and Rhesus typing. In 1985 WPBTS introduced routine screening for HIV antibodies. (Antibodies are specialized proteins our bodies produce in response to an invading organism) WPBTS uses the Abbott Prism, a fully automated state of the art test system. In 1996 an additional test for HIV was introduced to further improve blood safety. This is the P24 antigen test. P24 antigen is a protein, that is part of the HIV. Early in the infection, it is produced in excess and can be detected in the blood. The P24 test can detect HIV infection before the HIV antibody test can. Therefore, P24 antigen testing is used in diagnosing HIV early in the course of infection. The addition of the P24 antigen test further reduced the window period to about 14 days. In October 2005 NAT testing was introduced as a routine test on every donation.

Advantages of NAT

By testing for the nucleic acid we are able to detect very small amounts of the virus. This further reduces the window period which in turn has the desired effect of making our blood supply much safer. The test is extremely sensitive and incorporates state-of the-art NAT technology to detect viral RNA and DNA in donated blood during the very early stages of infection. WPBTS is using the Tigris - a fully automated test system. Use of the TIGRIS System allows individual donor testing, which is the most sensitive method of screening blood donations currently available. WPBTS will be using the antibody test plus NAT testing for the foreseeable future. The P24 antigen test will be dropped at the end of November 2005 as the NAT (which is more sensitive) takes its place.

Honesty of donors when answering questionnaires

Even though NAT testing represents an advance in terms of blood safety precautions through testing, it cannot safeguard blood safety 100%. We still have to rely on the honesty and integrity of our blood donors. We urge all donors to answer our donor questionnaire truthfully at every donation. Blood safety is a responsibility that donors must share with the Blood Service - we have to work together to ensure a safe blood supply.