DEAR DR. ROACH: In the coronavirus pandemic, many people are getting tested for 1) active illness and 2) antibodies. What’s the value of an asymptomatic person getting tested for an active infection? Do we even know if having antibodies gives you any protection? I’m reluctant to get tested for either if it can’t really alleviate the need to distance. What am I overlooking? — B.E.

ANSWER: You weren’t missing anything. There’s really no reason to be checking for the virus (the swab test is a PCR test looking for active infection, while antibodies look for past infection) unless you have a compelling reason to get a medical procedure or travel or could otherwise potentially expose others. In that case, a positive test would allow you to take stricter precautions.

Positive antibodies are necessary if you are enrolling in a study on the value of convalescent serum. Otherwise, it is mostly to satisfy curiosity about whether you were infected in the past or not. It remains unclear whether antibodies are needed for immunity; whether they confer immunity or not; and how long-lasting any immunity might be. Until these are known, getting antibody testing remains not very helpful.

DEAR DR. ROACH: I am writing for my 78-year-old sister. She had a vascular access port put in 16 years ago for non-Hodgkin lymphoma. She opted to keep it in, and gets it flushed every six weeks. In a recent column, you wrote that without flushing, the port could clot. Could this affect her in any way? Is it possible to remove after 16 years? — E.R.

ANSWER: Usually, a port can be removed fairly easily, although there are rare complications, especially air getting into the blood vessel. Many people, like your sister, choose to leave them in. Clotting of the catheter just means that the catheter can no longer be used to infuse fluid or chemotherapy; having a clot from the catheter break off and travel someplace — this is called “embolization” — is rare. Infection of the device is another rare complication. If she doesn’t mind going in every six weeks, continuing the flushing is safe, but removal is an option.

DEAR DR. ROACH: A few years ago, I developed a hemorrhage from the bowel. My local hospital was unable to do a transfusion due to my blood type, and I was transferred to another hospital. My bleeding stopped, and I never needed the transfusion. A few days after I left the hospital I received a letter saying I had anti-M antibody. What is it? Will I never be able to get a blood transfusion? — J.W.

ANSWER: The ABO blood system was the first one discovered, but the MNS system was discovered shortly thereafter. In the ABO system, if you give type A blood to a person with type O blood, you will get a severe transfusion reaction. This is because a person with type O blood doesn’t have the A protein. In the first few years of life, people with type O blood develop antibodies to the A protein.

The MNS system is similar. Most North Americans are type M, but development of anti-M antibodies is much less common than in the ABO system, so severe transfusion reactions are not seen as often. Most of the time, even people with anti-M antibody can get any blood that they are compatible with on the ABO system. The rare person with a history of a transfusion reaction may need M-negative blood.

I should note that a very experienced blood bank is necessary to interpret the anti-M antibody.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.

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