Plasma exchange or convalescent blood products are obtained by collecting whole blood or plasma from patients who have survived the target infection.
A brief history of plasma exchange
Plasma exchange or convalescent blood products are obtained by collecting whole blood or plasma from patients who have survived the target infection. When a person is infected their immune system reacts to the pathogen, recognizing its markers and producing antibodies against it. These antibodies can persist in the blood for a few months to years, and during a second infection, they can be made rapidly. This prevents the second infection from causing as much damage to the body, controls it, and eliminates it.
Transfusing plasma (the watery component of blood) from a patient with these antibodies into an infected patient will be effective in neutralizing the pathogen. Whilst antibiotics are useful in bacterial infections, viral infections usually have few or no drug therapies effective in neutralizing the virus. Plasma exchange can be used to help this subgroup of patients.
This is particularly valuable during pandemics as the infective pathogen usually emerges rapidly before we have any treatment for it. Plasma exchange offers a quick therapy as large volumes can be collected quickly, and it leaves the donor unaffected as their red blood cells are reinfused back, therefore preventing symptoms of anemia.
Plasma exchange was first used as far back as the 1880s, to treat bacterial and viral infections in animals and people. The first examples came from treating diphtheria produced from immunized animals, though human donors were quickly identified. The first time plasma exchange was used during a pandemic was during the Spanish influenza of 1918, with mixed results. From the 1920s to 1970s it was used to treat Scarlett fever, pertussis, measles, chickenpox, CMV, parvovirus. Horse serum was used to treat tetanus until the 1970s. More recent examples include SARS, MERS, and the H1N1 and H5N1 flu. It was particularly useful in cases where drug therapies didn’t exist.
Many studies treating these conditions show “consistent evidence for a reduction in mortality”, especially when the plasma was administered early in the infection. Unfortunately, many of the studies are low quality, with small study size, lacking a control group, and had a high chance of bias.
Ebola virus was a major cause of concern prior to COVID, and was responsible for two pandemics in the 2010 decade. Plasma exchange was first used to treat Ebola virus as far back as 1976. During the 1995 outbreak in Zaire, Africa, eight patients received 400mls of plasma, with seven surviving. The mortality rate in this group was just 12.5%, compared to 80% mortality in the general population, though with a sample size of just 8 the study is quite underpowered.
In 2014 the WHO published a guideline on the use of convalescent plasma during the Ebola Pandemic. This guideline promoted obtaining plasma from clinically asymptomatic survivors, 28 days after discharge from the hospital who have been tested negative twice for Ebola. This ensures a large group of healthy donors who would provide life-saving plasma for this fatal disease.
There are still some issues with plasma exchange. We still don’t have very high-quality studies on plasma exchange, we don’t know what risk are in transmitting infections to health professionals, the need for donors to provide the plasma, risk stratification when selecting donors, and the risk of transmitting other blood-borne infections (HIV, Hepatitis B, Hepatitis C and syphilis) between patients.
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