How Hospitals Test for Ebola

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Texas Health Presbyterian Hospital in Dallas, where the first case of Ebola virus was diagnosed in the United States. Brandon Wade / Reuters

Public health officials have announced that the first patient to ever be diagnosed with Ebola in the U.S. is being treated at a hospital in Texas. But how do you actually test to see if someone has the virus?

Thomas Ksiazek, a professor at the University of Texas Medical Branch at Galveston who has done extensive research on Ebola, says that testing is done using a process called real-time RT-PCR, or reverse transcription polymerase chain reaction.

In this technique, doctors or medical personnel take samples of blood from a patient. They then add an enzyme to convert RNA found in the blood into DNA (RNA is a chemical messenger that helps turn DNA’s “instructions” into proteins). Next, a “primer” is added that targets a string of genetic code unique to the Ebola virus. The concoction is then run through a PCR machine, wherein that strand of Ebola genetic material is amplified, or copied, many times (if it’s there, that is. If it’s not, nothing happens and the test returns a negative.)

Finally, a chemical probe is added that binds to these snippets of DNA and alerts the scientists to the presence of the Ebola virus, Ksiazek tells Newsweek. The whole process can take as little as three to four hours.

In the case of the man in Dallas, authorities initially didn’t connect the symptoms he presented with on September 26 to his recent visit to Liberia. When he returned to the Texas Health Presbyterian Hospital for a second time on September 28, he’d been sick for four days—certainly long enough for the PCR test to return a positive result, Ksiazek says.

In fact, the PCR test is sensitive enough that by the time most people show up with symptoms in a hospital, it will be able to detect the virus, says Ksiazek, who is a former  head of the CDC’s Special Pathogens Branch and has dealt with the containment of viruses like Ebola on a daily basis.

It should be noted that hospitals do not themselves test for Ebola, at least not in the United States at this time. They rather send samples when deemed appropriate to a local facility participating in the CDC's Laboratory Response Network. In the current texas case, hospital workers sent samples to the State Health Laboratory in Austin.

PCR machines are roughly twice the size of a typical desktop PC, and can be moved around relatively easily, though they do cost tens of thousands of dollars depending on the model, Ksiazek notes. These machines are present at some labs in Sierra Leone, Liberia and Guinea, but they require electricity to run—and in those countries, that often means unreliable gas-fueled generators. This fact, and the general lack of funding in some situations, has hampered efforts to get the outbreak under control, he says.

A technique called ELISA can also be used to diagnose Ebola, but it takes longer and also requires at least 100 times more individual viruses for an accurate result to be obtained than RT-PCR—meaning it is less sensitive, and doesn’t catch the virus as soon as the latter technique, Ksiazek says. The PCR technique was first widely used to diagnose Ebola in 2000.

Several groups of researchers are working on even faster, more portable tests, using disposable materials resembling home pregnancy tests, Science magazine reported last week. Two of the diagnostics will be tested in the coming weeks.

While the Ebola virus has the potential to kill hundreds of thousands in Africa, Ksiazek says he’s not concerned about it spreading in the United States—the CDC can quickly test for it and isolate patients who come down with the virus. Moreover, Ebola is not one of the more contagious diseases, in part because it requires close bodily contact to pass on. 

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