Nucleic acid testing

Nucleic acid test (NAT) lab performing tests on small pools of samples expected only viral marker antibody negative samples to be forwarded for testing. 

Nucleic acid amplification testing nucleic acid testing Nucleotide-binding domain... 

In order to increase the sensitivity of screening, so as to minimize the so-called window period during which serological markers will not detect the infectivity marker, methods of detecting nucleic acids from, for example, the virus particle have been developed. Hepatitis C virus nucleic acid testing is obligatory in Europe, the USA, and Japan, and in many countries nucleic acid testing for other viruses has also been implemented. There is nevertheless no doubt that in future PCR-based methods will further increase the safety of blood and blood products. 

Nucleic acid-based approaches are both sensitive and specific, and have been fielded as part of demonstrations and tests. Nucleic acid approaches cannot detect purified biological toxins but may be able to identify associated residues from the organism. The time required to perform a nucleic acid test is decreasing, and for some instruments it is now less than 10 minutes. Instrument packaging is also being dramatically reduced currently, suitcase-sized systems can be purchased. A nucleic acid approach to medical diagnostics is also showing promise... 

CT and GC will be discussed together since several of the available nucleic acid tests for these pathogens are multiplex assays. Though both CT and GC can cause a variety of cflnical infections, we will focus on genital infections. 

The standard methods for detection of MTb include acid-fast bacilli (AFB) smear and conventional and liquid culture methods. The AFB smear is rapid, but has a poor sensitivity of 20% to 80%. Another challenge with the AFB smear is that it cannot distinguish MTb from nontuberculous mycobacteria (NTM), such as M. avium-complex (MAC). This distinction is important because disseminated MAC and MTb are both common infections in persons with AIDS. Culture methods for the detection of MTb are sensitive, but growth detectable by standard methods may require 6 to 8 weeks in a culture. Growth often occurs more quickly in liquid culture than with conventional methods, but can still require weeks. With these limitations of culture methods, there was great enthusiasm for nucleic acid testing as a rapid, sensitive method for detection of MTb, especially given the needs to rapidly isolate patients with active, untreated disease and to initiate prompt therapy, particularly in immunocompromised hosts. 

Both the MTD test and the Amplicor test have a sensitivity of 95% to 98% when using APB smear-positive respiratory specimens, and the specificity ranges from 99% to 100%. However, early studies showed that the sensitivity for APB smear-negative specimens was -50%. Based on these data, it was clear that the test could not he used to rule out MTb infection on smear-negative respiratory specimens. This further limited the clinical utility of these tests, because it became clear that the nucleic acid testing would be used to supplement the AFB smear and culture rather than replace these testing modalities. Another limitation of the currently available nucleic acid assays is that they can be used only on specimens from patients who had not received antituberculosis therapy within the past 12 months. This limitation was included because DNA can persist in respiratory secretions (and other body fluids) for months after the mycobacteria are no longer viable. 

Though there are considerable limitations to the currently available nucleic acid tests for the detection of MTb infection, they have a role in the clinical laboratory in that they can provide a rapid diagnosis. The goal would he to develop assays with a higher sensitivity (>99%) for AFB smear-negative specimens, because this may eliminate the need to culture specimens that are negative when tested in nucleic acid assays. 

S. Chen, G. Selecman, and B. Lemieux, Expanding rapid nucleic acid testing, IVD Technology, vol. 7, p. 51, 2004. 

Simonsen, M., Nucleic acid testing, proteomics to drive future of diagnostics, BBI Newsletter, 27, 221-228, 2004. 

Zou, S., Dorsey, K.A., Notari, E.P., Foster, G.A., Krysztof, D.E., Musavi, F., Dodd, R.Y., Stramer, S.L., 2010. Prevalence, incidence, and residual risk of human immunodeficiency virus and hepatitis C virus infections among United States blood donors since the introduction of nucleic acid testing. Transfusion 2010 (50), 1495-1504. 

Nubling CM, Urtkelbach U, Chudy M, Seitz R (2008) Effect of viral nucleic acid testing on crurtamination frequency of manufacturing plasma pools. Transfusion 48 822-826... 

Nucleic acid tests have been steadily increasing in their importance and in the market share of all IVD tests over the last decade. They cover a range of different clinical applications from infectious diseases, in which they are establishing themselves as the gold standard for many tests, to new genetic tests for applications ranging from... 

Plasma-derived factor concentrates undergo viral inactivation steps however, the nonenveloped parvovirus B19 is resistant to these treatments. Since implementation of nucleic acid test screening for parvovirus B19, receiving these concentrates continues to be associated with increased risk of developing antibodies to parvovirus B19 (OR 1.7, 95%Cl = 1.2-2.4) [162 ]. 

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