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Your Good Health: 'Mixing study' reveals whether high aPTT needs treatment

The activated partial thromboplastin time (aPTT) is a measurement of blood clotting
Dr. Keith Roach

Dear Dr. Roach: I recently had a surgery, and prior to the surgery, my doctor ordered some routine blood tests. My aPTT level was high, but the PT level was normal. The doctor said it was probably nothing, and the surgery went fine. But, when I went back for my annual checkup, he rechecked it, and it’s still high. Do I need to worry?


The activated partial thromboplastin time (aPTT) is a measurement of blood clotting. A prolonged aPTT with a normal prothrombin time could mean a deficiency of one of the clotting factors (proteins that help make a mature blood clot), either by an inhibitor of one of the blood-clotting proteins, or by a condition called von Willebrand disease. Many of these factors are not associated with risk of any conditions, such as excessive bleeding. However, the elevation of aPTT could indicate an increased risk for bleeding (and even abnormal clotting in the confusingly named “lupus anticoagulant”).

The next stop is usually a hematologist, who will perform a “mixing study,” where your blood is mixed with normal blood, and then retested to see if that makes the aPTT level come out normal. If it doesn’t, it’s likely you have an inhibitor. If the mixing study corrects the aPTT level, you have a protein deficiency, since even 50% of a blood-clotting factor is enough to have clots normally and have a normal aPTT. Further testing will help decide whether this is something that will require treatment.

Dear Dr. Roach: My husband and I are of the generation old enough to have received small pox vaccinations as children. So, do we, therefore, carry immunity to monkeypox?


Monkeypox is closely related to smallpox, but originally was found in monkeys. Currently, there is a worldwide outbreak of monkeypox, with thousands of cases in the U.S. (especially in New York City where I practice, but now in almost all states), but also high amounts in Europe and in smaller amounts across the globe. Monkeypox starts as a few blisters, then develops larger numbers. Most people with monkeypox find it quite painful, but it isn’t always painful. It is usually spread by close contact, such as sexual contact, but it doesn’t need to be sexual contact at all. Unlike the virus that causes COVID-19, monkeypox can last a long time on surfaces. Monkeypox can also be spread through respiratory droplets, but it is not very infectious. It usually requires people to be face-to-face for a while to transmit disease.

In previous outbreaks of monkeypox, it was found that people with a distant history of a smallpox vaccination had a much lower rate of developing monkeypox than those who had not had the vaccine: One estimate concluded that the vaccine was 80% effective at preventing disease. (That 80% number is debated, since the previous outbreaks were quite a few years ago.) So, you probably have some — but not absolute — immunity to monkeypox, because you were vaccinated. (Vaccination stopped in the U.S. in 1972, but continued into the 1980s in some other countries.) Most vaccinated people still have their smallpox vaccination scar.

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

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