Monday, November 17, 2014

Referral: Mackay On Early Negative Testing For Ebola

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Credit CDC PHIL

 

NOTE:  As I am posting this, NBC News is reporting the death of Dr. Martin Salia, who reportedly initially tested negative for the virus even after becoming symptomatic.

Ebola Patient Dr. Martin Salia Has Died in Nebraska: Officials

Dr. Martin Salia, an Ebola patient being treated in Nebraska, has died, medical officials said Monday.

Salia was diagnosed with Ebola in Sierra Leone and was airlifted to Nebraska on Saturday. He was the 10th Ebola patient to be treated on U.S. soil and the third treated at Nebraska Medical Center.

 

# 9337

 

We’ve often heard that Ebola tests aren’t conclusive until they are taken 48-72 hours after symptoms first appear, and based on recent media reports (see WaPo’s A doctor’s mistaken Ebola test: ‘We were celebrating. . . . Then everything fell apart’), that appears to have been the case with Dr. Martin Salia who died this morning.

 

Dr. Ian Mackay gives us some deep background on the testing for Ebola, and why a 48-72 hour window for testing was selected.

 

Ebola testing: 48-72 hours for a negative to turn positive

Currently, some fraction of the people who present very early after they may have been infected by Ebola virus for testing, return a negative result. This is probably a rare event because the majority of cases arrive for care with Ebola virus disease (EVD) already well underway.


The latest Centers for Disease Control and Prevention (CDC) guidance in these instances is to wait (48 to) 72 hours and see if the patient remains ill, or becomes more unwell. If they do either of these, a second test is performed.[1] If the suspect case recovers from illness, no repeated testing is indicated.


The test we rely on to confirm a clinically suspected EVD case is called a reverse transcriptase polymerase chain reaction (RT-PCR). RT-PCR is a technique designed to  seek out a tiny but very specific region of the Ebola virus's RNA genome, copy it into DNA then amplify those DNA copies a billion-fold by making more copies. Somewhere during that exponential amplification process, the technology of the day (currently fluorescence detection but formerly agarose gel detection, radiation and chemiluminescence) allows us to identify that the specific DNA we seek is appearing above an arbitrary threshold...we have a positive test result for Ebola virus.


(Continue . . . )

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