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The answer is a bit more complicated.  If there is no target for %S, there is no need to test for hemoglobin S in donor blood.  In other words, if the transfusion is purely for anemia, not treatment of acute chest syndrome or prevention of stroke, there is usually no target %S being used by the treating physician.  The reason for testing for S in the donor is not the risk to the patient, but because transfusing S containing blood can confuse the calculation of % S overall. It's probably unnecessary because the %S contribution of a single unit of S heterozygous red cells in an exchange transfusion or red cell apheresis is very small.  S hemoglobin in a red cell that is from a heterozygous donor does not contribute to sickling complications.  This is well known because individuals who are heterozygous for S do not have complications of sickle cell disease.  Thus while it is traditional to test for hemoglobin S in donor blood for sickle cell recipients, this is probably unnecessary unless the treating physician is trying to achieve a specific %S, as in stroke prevention.

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Just a side note that came to mind while reading this post.  Do you still give Hgb-S negative blood for neonate transfusions? Back when I was still in the world we did so I was just wondering if it was still in vogue.  If I remember correctly, the theory was to make sure we were not compromising the oxygen carrying ability for the neonate.  Didn't mean to hijack David's post.   :coffeecup:

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People heterozygous for hemoglobin S have no clinical level problems with transporting oxygen, so there is no need (except for being overly cautious) to provide hemoglobin S tested blood for newborns,  or, as I mentioned above,  anyone who is not being monitored for % S.  Another one of those "it seemed like a good idea at the time" but not evidence based approaches.

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On 6/8/2020 at 7:16 AM, Neil Blumberg said:

People heterozygous for hemoglobin S have no clinical level problems with transporting oxygen, so there is no need (except for being overly cautious) to provide hemoglobin S tested blood for newborns,  or, as I mentioned above,  anyone who is not being monitored for % S.  Another one of those "it seemed like a good idea at the time" but not evidence based approaches.

I hope you can clarify some stories I have heard.  We live at 3600' elevation with a lovely ski hill that is around 8000' (see my profile photo).  We have had people come here to ski and be surprised at having sickle problems (I assume sickle crisis). Does this mean these are all homozygous HbS people who just don't have much trouble at lower elevations?  I also heard a story from Sarah Ilstrup(sp) that a group of young black professionals took a ski vacation to her Utah mountains and many of them were surprised by developing sickle symptoms.  We have little experience here with sickle disease because those with overt disease usually don't live at this elevation.  I would love education on the genetics and Hbs percentaage status of these patients if you would care to speculate.  We also get HbS negative units in for neonatal exchanges.  Is that more necessary here than at sea level?

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Patients with sickle trait do not have sickle crises at all, except in very rare cases when they become seriously hypoxic. There are rare cases reported in military personnel and athletes.  It's not clear whether they are sickle crises in some cases or just similar symptoms seen in non-sickle cell patients.   These are truly rare cases, and usually at higher stress than an altitude of 8,000 feet, which really isn't terribly high.  No personal experience.  In extreme circumstances, patients heterozygous for S may be slightly more susceptible to symptoms like mountain sickness, just like everyone else.  Don't know for sure. They have lower hematocrits and this may be one factor.  Distinguishing between sickle cell symptoms and mountain sickness may be difficult for some clinicians.  Lots of people get sick at altitude, especially if they exert themselves or make the ascent rapidly, or are dehydrated. I would guess that is what happened to this group. 

 

There is no need for hemoglobin S negative units for routine transfusions to non-sickle cell, hemoglobin homozygous patients.  

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Should add that individuals with sickle trait have % S that is usually less than 50%.  At those levels, even patients with SS disease usually are asymptomatic.  Sickle trait individuals do not have symptoms under anything less than extreme physiologic stress, although some believe that occasional individuals have had complications when severely hypoxic or stressed.  These case reports tend to neglect the fact that individuals who do not have S hemoglobin can have similar symptoms under extreme physiologic or hypoxic stress, so it's not clear these individuals with sickle trait are truly at greater risk.  Larger cohort studies have sometimes suggested a higher rate of symptoms among AS (sickle trait) individuals, but there remains some doubt, at least in my mind :).

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