RBC tagging question

[FotoFusion]

A couple of questions for you radiology types. I've been reading a little bit about tagged RBC studies for detecting GI bleeds, but I didn't understand a couple of things.

When you tag RBC's with technetium, what part of the RBC is actually being tagged? The membrane? Hgb? Some other cytoplasmic protein?
Which vein is the labeled blood injected into?

I'm not asking you to do my work for me, I did the rest of the reading but I'm curious about these things, that's all 🙂 Thanks.

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[f_w]

When you tag RBC's with technetium, what part of the RBC is actually being tagged? The membrane? Hgb? Some other cytoplasmic protein?

http://www.imaging.mallinckrodt.com/_Attachments/PackageInserts/Ultratag RBC PI.pdf

 

[FotoFusion]

http://www.imaging.mallinckrodt.com/_Attachments/PackageInserts/Ultratag RBC PI.pdf

Thank You!! 😎

 

[pappadoppleus]

That link was dead, but I found this. Apparently you can inject the 99mTc IV after pre-treatment with Tin Pyrophosphate (Sn2P2O7). The Sn2+ goes into RBC first and then you give the technetium which diffuses into the RBC. Apparently this doesn't tag very well, so Mallinckrodt developed an in vitro RBC tagging system where you take a few mL of non-coagulated blood and add the 99mTc in vitro and then inject those labeled cells IV. From what it looks like though, that in vitro kit was removed from the market. Read for yourself if you'd like...

http://www.nucmedtutorials.com/dwcelllabel/cell1.html

I'm def not a radiologist, just trying to make sense of this stuff while studying.
-PD

 

[fun8stuff]

That link was dead, but I found this. Apparently you can inject the 99mTc IV after pre-treatment with Tin Pyrophosphate (Sn2P2O7). The Sn2+ goes into RBC first and then you give the technetium which diffuses into the RBC. Apparently this doesn't tag very well, so Mallinckrodt developed an in vitro RBC tagging system where you take a few mL of non-coagulated blood and add the 99mTc in vitro and then inject those labeled cells IV. From what it looks like though, that in vitro kit was removed from the market. Read for yourself if you'd like...

http://www.nucmedtutorials.com/dwcelllabel/cell1.html

I'm def not a radiologist, just trying to make sense of this stuff while studying.
-PD

research the in vitro "ultratag" method...

 

[cowme]

Is there really any reason to ever order this study anymore? If you cant see a bleed on a scope or angio....who really cares?

 

[colbgw02]

Is there really any reason to ever order this study anymore? If you cant see a bleed on a scope or angio....who really cares?

Yeah, there is plenty of reason to order this study.

For lower GI bleeds, a tagged-RBC study is appreciably more sensitive than angiography, about 10 times so. It can detect bleeding as slow as 0.1 cc/min, as opposed to about 1 cc/min for angio.

As far as endoscopy goes, if the source of the bleeding is obvious, then clearly you don't need this study. If it's BRBPR, then the patient will often go straight to colonoscopy. It's not always that straightforward though. For a lot of lower GI bleeds, the endoscopy turns out to be negative, in which case the nucs study is very helpful.

Plus, repeated runs on angiography looking for and subsequently treating a small but hemodynamically significant bleed isn't exactly a benign treatment. The interventionalists that I know won't even get out of bed to treat a lower GI bleed until the scintigraphy is done. Then, even if they go to angio and don't see the bleed, they can coil in the expected vascular distrubtion based upon the scintigraphy.

 

[cowme]

Interesting. At my med school, I don't think I saw the study ordered once, and I had my fair share of LGIB patients. Even on my week of Nuc medicine, I didn't see the study once. On the other hand, I saw tons of hemodynamically stable patients go straight to IR with nothing more than heme + stool and hemoglobins of 11. The hospitalists and IR guys seemed to think it was a pretty useless study, but maybe its very program dependent (and maybe I just didn't see it as much as I would have with other attendings)

Also, How often do u guys coil based on what you see on the tagged rbc scan without seeing a bleed on angio? Do bleeds that can only be visualized on rbc scans really make patients hemodynamically unstable that often?

 

[colbgw02]

Interesting. At my med school, I don't think I saw the study ordered once, and I had my fair share of LGIB patients. Even on my week of Nuc medicine, I didn't see the study once. On the other hand, I saw tons of hemodynamically stable patients go straight to IR with nothing more than heme + stool and hemoglobins of 11. The hospitalists and IR guys seemed to think it was a pretty useless study, but maybe its very program dependent (and maybe I just didn't see it as much as I would have with other attendings)

My (cynical) suspicion is that the interventionalists are more than happy to skip the tagged-RBC study for reimbursement reasons. Why worry about avoiding an unnecessary angiogram when you can bill for it? But I digress.

Also, How often do u guys coil based on what you see on the tagged rbc scan without seeing a bleed on angio?

It's operator dependent, in my experience. I've seen people do this without a second thought, especially if they're using Gelfoam, which will dissolve over time. Other interventionalists I know wouldn't do this unless the patient is circling the drain.

Do bleeds that can only be visualized on rbc scans really make patients hemodynamically unstable that often?

Absolutely. When the bleed is into a large potential space (hip joint, lung, abdomen, etc.), it doesn't take much of a bleed to get the patient into trouble. There's also the issue of timing. An angiographic run usually lasts less than 10 seconds. If the bleed is large but intermittent, then there's a very good chance it'll be missed on angiography. Scintigraphy has the advantage of imaging over a longer time period, especially if Tc-99m sulfur colloid is used instead of Tc-99m labeled RBCs, enabling it to catch bleeds that come and go.


It's definitely a niche study; it's not like every GI bleed patient wandering into the ER will get this study. It definitely still has its place though.

 

[cowme]

My (cynical) suspicion is that the interventionalists are more than happy to skip the tagged-RBC study for reimbursement reasons. Why worry about avoiding an unnecessary angiogram when you can bill for it? But I digress.

I guess this is the one of the benefits of training at different hospitals over your career, lol. I always thought the study was only one of historical purposes and for USMLE tests. Thanks