False Negatives Muddy the Waters for Ebola Detectives

[A] survey of recent news here in the United States, reveals a growing number of ‘catch and release’ sorts of stories. Nigerian passenger on airline who vomits, but who tests negative for Ebola; a child who has just returned from West Africa vomits on an airline in Chicago, who then tests negative; a man with a nosebleed who lands at Washington Dulles, who then tests negative. You get the picture. Well, this morning we have the story of a female health care worker who has just returned to the US after working with Ebola patients in West Africa and is put in a holding room according to brand new New York and New Jersey rules for incoming passengers who might pose a risk. The woman developed a fever, but she has now tested negative for Ebola. Is this good news?

Perhaps. Perhaps not. You see, it is quite possible for an Ebola patient to test negative but later test positive.  Currently, the ‘gold standard’ test uses PCR amplification of a small amount of blood from the patient. PCR stands for ‘polymerase chain reaction’, and it is an elegant machine that makes thousands of copies of a tiny fraction of DNA–or in this case, it’s looking for RNA within the blood. Ebola is a negative sense RNA virus, and an infected person should have lots and lots of that RNA in his or her blood. But not always.

A study in 2002 showed that false negative results can occur in some patients:

False-negative RT-PCR results are likely to occur for patients with severe viral hemorrhagic fevers, especially in the acute phase of the disease where a rapid confirmation is required. Their plasma may contain large amounts of RT-PCR inhibitors, probably resulting from the decay of tissue. These inhibitors can be detected by control reactions with spiked samples (low copy numbers of control RNA, 1 log10 above detection limit of the PCR) (5). Control reactions to detect inhibitors of RT-PCR are mandatory for a safe diagnosis for patients with suspected VHF [Viral Hemorrhagic Fever].

I pray that every negative test is a correct result, but wouldn’t it be prudent of the health profession to conduct additional tests over a period of days before releasing someone who may actually be infected? Just my two cents.