When evaluating a specimen for DAT testing, our practise has been to accept a specimen up to 24 hours old, refrigerated if not tested within a couple of hours of collection is preferred.  Some of us have been taught that older specs may yield a false positive or a false negative; others just a false positive.  We would like to clarify our procedure.

 

I would appreciate any guidance on specimen age and impact on DAT results. What is your practise?  Can you point me to any references?

 

Thanks!

 

I know that the manufacturer instructions (for our reagent) indicate that the specimen is acceptable for 72 hours as long as it's been refrigerated.

 

That's a good question.

Edited September 19, 201410 yr by goodchild

Hi bb4me,

 

I think that this may be a "hang over" from the days of using clotted samples for blood transfusion departments.  In those days, of course, the samples were taken into dry, glass tubes.  In such tubes, if the samples were stored for any length of time, the complement cascade could be started and then stalled at C3d, but in a non-specific way, so that you got a false positive result.

 

Now that we use EDTA samples, and EDTA chelates Ca⁺⁺, Mg⁺⁺ and Mn⁺⁺, all of which are required for the initiation of the classical complement cascade, this is no longer a problem

I know that the manufacturer instructions (for our reagent) indicate that the specimen is acceptable for 72 hours as long as it's been refrigerated.

 

That's a good question.

Thanks, Goodchild.  We have multiple methods (gel and tube), so I will be sure to consult all applicable inserts.

 

Hi bb4me,

 

I think that this may be a "hang over" from the days of using clotted samples for blood transfusion departments.  In those days, of course, the samples were taken into dry, glass tubes.  In such tubes, if the samples were stored for any length of time, the complement cascade could be started and then stalled at C3d, but in a non-specific way, so that you got a false positive result.

 

Now that we use EDTA samples, and EDTA chelates Ca⁺⁺, Mg⁺⁺ and Mn⁺⁺, all of which are required for the initiation of the classical complement cascade, this is no longer a problem

Thanks, Malcom.  Yes, the "glass factor" was always my favorite.  We also recognize that a positive C3b/C3d result from a refrigerated serum spec might be questionable, and recommend retesting using an EDTA sample, especially in the absence of historical results.  That being said, we rarely see serum samples in our Transfusion Service any more.

Edited September 19, 201410 yr by bb4me