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Calculating the Frequency for finding antigen negative unit


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Please help me in calculation: Need to find one antigen negative unit for patient whose blood type is B positive. Need C-, K-, Fya- and Jkb- unit. We want to use only B negative and O negative units so I want frequency based on that.

Please demonstrate how you got the answer Please...Thanx

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Bear in mind that the supplier you're getting your blood products from is also snatching up a fair number of the antigen-negative blood products. As an example this means that the percent of blood donors negative for Fy(a) can't be reliably used to determine a frequency calculation.

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Very very roughly, and assuming your donors are mostly White European

If you start off with 100 units, approx. 50 will be O or B

Of those approx. 5 will be K+ - so you will have 45 units.

Of those, about 2/3  (so approx. 30) will be Fya+, so you will have 15 left

Of those about 80%  (about 12) will be Jka+

So, if you're VERY lucky, the remaining 3 will be compatible.

Good luck!!!!!

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Assuming you don't want to count the frequency of B or O and just want to know how many B or O neg units you need to screen it should be 0.34 X 0.25 X 0.9 X 0.99 or so (frequencies from memory) which gives you 7.5%.  If you screen about 15 units, you should find one--unless the blood supplier has kept all of those negative units or you have bad luck.  Statistics don't hold up well for small numbers so you might screen 30 before finding 1, then will find 3 in the next 30.  Hope I got that right.

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Standard text books on blood banking describe the method of calculating the number of units required to be screened before antigen / antigens negative units can be found. For this you should know the frequency of each antigen in the given population. I have given below an examble.

 

Step I

 

Express the negative frequency of each antigen in a given population as a fraction of one in two decimal points and multifly them all and convetrt the resulting fraction into a percentage by multiplying by 100.

 

0.18( c ) x 0.34(Jka) = 0.06 x 100 = 6%

 

Step II

Then divide 100 by this number.

 

100/6 = 16 units

 

 

 

 

 

 

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When you start off with the neg units most of them will be C-.

So I thought you need to screen less units .

Malcolm please give you input. Tx

Sorry Eagle Eye, and I KNOW that this sounds terribly egocentric, but I don't bother with working things out like this any more. After a while, you sort of "remember", and you sort of "know" how common or how rare a particular combination will be, and you just go from there.

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If you used the general frequency of C in the population it would say you need to screen more units.  I used the approximate frequency of C in Rh neg people (or lack thereof, really) in my calculation. Using unselected Rh units you would have to screen about twice as many than if you start with Rh neg.

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I kind of agree with Malcolm, above (usually a safe bet).  Excercises like calculating probabilities of getting a compatible unit for a particular antigenic profile seems a bit silly.  (On the other hand, we still have our Hematology students do a few cell counts for CBCs on a hemacytometer just "for the experience".)

 

Scott

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I wish I could get my techs (generalists) to do the math, but they are insecure (and stubborn!). They are very likely to screen 6-10 units automatically whether they are looking for Jka negative or K negative units. If they are looking for a unit negative for multiple antigens, they'll screen all 6-10 units for every antigen, instead of screening for them one antigen at a time. Quicker their way maybe, but antisera is toooo expensive to play that game.  I show them the frequency chart and show them the calculation, then scream when they do it their way again next time. Argh!!!!!!

 

We have a patient with anti-Bga, -K and -Cw right now who gets one unit a week (screen currently negative for K and Cw). I had to write specific instructions about screening units for her - crossmatch to find a unit compatible for the Bga antibody, then screen 1-2 of the compatible units for K and then screen 1 K neg unit (and only 1 unit) for Cw. If it's Cw pos, screen 1 (and only 1) more. Otherwise they were screening 6-8-10 units for Cw and wasting antisera that costs $900+ for a  teeny tiny bottle!

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I am going to be EXTREMELY controversial here, but I really don't understand why, if the cross-match is compatible, anyone would bother to type for the Cw antigen. I would be happy if someone could direct me to a RELIABLE paper that has shown anti-Cw to be clinically significant as far as an acute or delayed haemolytic transfusion reaction is concerned. I am aware of one paper in which it was claimed that anti-Cw caused hydrops in a pregnancy, but, for reasons I have given before, I do not regard this paper as reliable.

The same goes for an anti-M that is not proven to work at strictly 37oC, and many other specificities.

One only has to read the three editions of the FactsBook to see that there are numerous antibody specificities that are absolutely benign (unless your name is Lyndall Molthan - see Issitt PD. Applied Blood Group Serology. 3rd edition, 1985, Montgomery Scientific Publications, page 433. Anyone who cannot get hold of this is welcome to read my signed edition, but you'll have to give me £1 million as security, so that I get it back!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

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Malcolm, if you have good luck lending out your Applied Blood Group Serology for big bucks, please let me know. I have a copy I'll rent for the same price.   :excited:

 

As for the anti-Cw, hmmmmm. My blood service reference lab recommends screening for it. I'll do some reading.

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I would thoroughly recommend Daniels G, Poole J, de Silva M, Callaghan T, MacLennan S, Smith N.  The clinical significance of blood group antibodies.  Transfusion medicine 2002; 12: 287-295.  This states that for all Rh antibodies EXCEPT ANTI-Cw, one should give antigen-negative red cells.  For anti-Cw, it is recommended that red cells compatible by IAT at 37oC should be given.

 

In the body of the paper, the authors state, "Anti-Cw is a relatively common antibody.  THERE IS NO REPORT OF ANTI-Cw CAUSING A TRANSFUSION REACTION, and IAT-compatible blood may be selected."

 

I am concious that, in the FactsBook, the authors state that the clinical significance of alloanti-Cw, Transfusion reaction:  Mild to severe; immediate/delayed, but I am also concious that they give no citations whatsoever for this statement.

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  • 7 years later...

I will attempt to get the Non ABO Rh antigen units (C-0.2, K-0.91, Fya- 0.34, Jkb- 0.28) C*K*Fya*Jkb gives 0.0173 which gives number of units to be screened as 1/0.0173 or 57.8 or roughly 59-60 units to get ONE antigen negative RBC unit.

If you want to consider the ABO Rh then O Rh Neg is 0.40*.15 or 0.06. multiply this with the former will give 1/0.00104 or 964 units!! scary. good luck

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I tend to work them the way "galvania" works them too.  I just start with the high frequency antigens and work down to the low frequency ones.  That is the way we screen too - eliminate the high frequency antigens and screen only the negative units for the lower frequencies as you get to each antigen.  (mostly we just call our distribution center (Vitalant, El Paso) for units like that - they are doing an outstanding job getting units for the "messy" patients!!):)

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  • 1 month later...

All the talk about statistics is great but in the real world you never know:  I once screened over 30 units for K.  All were positive.  As I was the night guy, the day folks were laughing until they got the same results.  All we could figure is the blood center was screening for K and shunted all the +s to  a shelf which we received in bulk.  I've also screened for Fya in past.  Once i screened 4 units and found 2.  The next time I had to screen 16 and the last 2 were  negative.  As I said, the stats look good but reality is sometimes a bit different.

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11 hours ago, David Saikin said:

All the talk about statistics is great but in the real world you never know:  I once screened over 30 units for K.  All were positive.  As I was the night guy, the day folks were laughing until they got the same results.  All we could figure is the blood center was screening for K and shunted all the +s to  a shelf which we received in bulk.  I've also screened for Fya in past.  Once i screened 4 units and found 2.  The next time I had to screen 16 and the last 2 were  negative.  As I said, the stats look good but reality is sometimes a bit different.

I learnt that when screening to take units distributed evenly throughout our inventory to prevent the situation David found himself in, after I screened the first 15 units on the self (shortest dates) for E, which were all positive! The units after were conformed to the expected frequency. This situation occurs because reference labs will routinely batch screen for single antigen or basic combination requests from the blood center's general inventory to prevent or at least minimize use of the reference lab's rarer units.  

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7 hours ago, Ensis01 said:

I learnt that when screening to take units distributed evenly throughout our inventory to prevent the situation David found himself in, after I screened the first 15 units on the self (shortest dates) for E, which were all positive! The units after were conformed to the expected frequency. This situation occurs because reference labs will routinely batch screen for single antigen or basic combination requests from the blood center's general inventory to prevent or at least minimize use of the reference lab's rarer units.  

Not so in the UK.  ALL of our units are typed for the D, C, c, E, e and K antigens, as well as (obviously) ABO and mandatory viral markers.  However, those that are intended for patients with atypical antibodies will come from a store of units that are further "fully typed" (usually from the MNS to the Kidd BGS) and these tend to be all K Negative, as they are intended for individuals who have already shown themselves to be "responders", most of whom will be K Negative, and K is a highly immunogenic antigen (although, of course, those intended for individuals with anti-k will be K Positive and k Negative!).  The number of these typed units is not, however, immense, and so does not really "skew" the overall antigen frequency in the normal stock.

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I agree with those who 'don't bother' with the actual math ... between 'natural selection' and blood suppliers 'holding' certain antigen types, exact math is just an academic exercise. 

To be practical (considering tech time and reagents are valuable commodities):

  • If the patient's plasma contains demonstrable antibody, crossmatch a batch or two of units then do the antigen typing on the compatible units only.  No luck = order antigen-neg from the supplier.
  • If the patient's plasma is negative, then screen (highest frequency first) a batch or two of units.  Again, No luck = order antigen-neg from the supplier.

 

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19 hours ago, Malcolm Needs said:

Not so in the UK.  ALL of our units are typed for the D, C, c, E, e and K antigens, as well as (obviously) ABO and mandatory viral markers.  However, those that are intended for patients with atypical antibodies will come from a store of units that are further "fully typed" (usually from the MNS to the Kidd BGS) and these tend to be all K Negative, as they are intended for individuals who have already shown themselves to be "responders", most of whom will be K Negative, and K is a highly immunogenic antigen (although, of course, those intended for individuals with anti-k will be K Positive and k Negative!).  The number of these typed units is not, however, immense, and so does not really "skew" the overall antigen frequency in the normal stock.

The US has many different organizational blood suppliers. While some organizations are national like the American Rec Cross (ARC) there are many regional and even local organizations. In my experience, each center has their own screening policy, which is determined by their hospitals requirements. A region with a high sickle cell population may send all (or most) new African American donors for molecular testing, while other regions may only screen units when specificities are needed.

So, when a blood center has an aggressive screening policy, or when they are looking for specific phenotype may affect the frequencies hospitals encounter. This explains Cliff’s and my experience described above. Also, the local donor population, and/or if (and when) the blood center imports units from a different region may impact the antigen frequencies hospitals encounter .

As described by other posters above, I use the antigen frequencies to primarily determine the order in which to screen antigens and to manage expectations. For example, when I need to screen for R2R2 K- units there is an approximate expectation of 2%. I would therefore screen batches of 100 units; on one occasion I found zero units, on another 9 units with the norm being between 1 and 3 units. I am jealous screening for R2R2 K- units (or any Rh combo) would not be needed in the UK!!

 

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  • 2 weeks later...

Here is an example of how to calculate numbers of units to be screened from a Proficiency test we designed a couple of years ago.  It makes a nice worked example.

 

Frequencies and Donor Screening

The frequency of the c (small) antigen is 80% in Caucasians, 96% in Blacks, and 47% in Asians (Refer to Table 3 for additional information regarding Rh antigen frequencies in various populations).3

 

Antigen Symbol

Caucasian

Black

Asian

D

85%

92%

99%

C

68%

27%

93%

E

29%

22%

39%

c

80%

96%

47%

e

98%

98%

96%

Table 3: Common Rh blood group antigen frequencies3

 

The frequency of the K antigen in 9% in Caucasians and 2% in Blacks. The K antigen can be found at higher frequencies in specific populations—found in approximately 12% of Iranian Jews and can be found in up to 25% of the Arab population (Refer to Table 4 for additional information regarding the antigen frequencies associated with the Kell blood group system).

 

Antigen Symbol

Caucasian

Black

Other

K

9%

2%

Iranian Jews = 12%

Arabs = As high as 25%

k (small)

99.8%

100%

 

Kpa

2%

Less than 0.01%

 

Kpb

100%

100%

 

Jsa

0.01%

20%

 

Jsb

100%

99%

 

Table 4: Common Kell blood group antigen frequencies3

 

Problem #1: How many units of pRBCs should be crossmatched in order to find 3 antigen negative units?

 

1.      Given that ~80% of the population will have the c (small) antigen, ~20% of the donors should be negative for the c (small) antigen. Additionally, given that ~9% of the population will have the K antigen, ~91% of donors should be negative for the K antigen.

**NOTE: When multiple antigen frequencies are involved, you must multiply the antigen negative frequencies together. In this case we would multiple 20% and 91% to account for the donors who are c (small) negative and K negative.

2.      Number of units to be crossmatched = x

 
 

 

 

 

x = 3 units/0.18 = 16.7 (or 17 units)

 

3.      The blood bank should crossmatch 17 units, where 3 of the 17 units should be compatible.

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