Addressing 'order of draw' in comparative calcium levels. (Letters to the Editor).

I read with interest the answer given in the July MLO "Tips from the clinical experts" column, concerning comparative calcium levels from one hospital clinical lab to the next. One possibility (and the easiest to investigate) has not been addressed in this question -- order of draw in the collection process.

Several years ago, our institution did a study on calcium levels and order of draw. (1) It was determined that the difference in the calcium levels drawn at the beginning of a collection and those collected at the end were significant enough to have an impact on the clinical management of the patient. The individual submitting the question needs to investigate what each site is observing for order of draw. With venous occlusion and time, such analytes as calcium and total protein will be greatly affected.

When establishing an order of draw for blood collections, one needs to look at the additives and the tests to be collected (physiological factors). The order of draw that we observe is to have chemistry (especially calcium) collected first when using the evacuated system, or if using a syringe, it must be dispensed from the first syringe (if multiple syringes on a butterfly necessary). Our order of draw when collecting by the evacuated system is: gel separator tubes, red-topped, Citrate, Heparin, EDTA, Fluoride/Oxalate. If collecting and dispensing from a syringe, our order of draw is: Citrate, Heparin, EDTA, Fluoride/Oxalate, gel-separator, and red-topped. We realize this differs from the latest NCCLS recommendations, but feel strongly about calcium issues and will continue to use our existing order of draw. Our order of draw more closely resembles the NCCLS guideline of H3-A3 (1991).

Even though the gel-separator has an additive acting as a clot accelerator, it should not be put after the Citrate as outlined above. This past June, we conducted a phlebotomy conference at our institution. From talking to individuals from all over the United States and Canada, the order of draw observed seems to be as varied as the institutions represented at the conference. I would refer you to the following esources:

1. J.D. Jones. Journal of Occupational Medicine (May 1980), Vol. 22, No. 5, p. 316-320.

2. H. Husdan, A. Rapoport, S. Locke,& D. Oreopoulos. Clinical Chemistry (1974), Vol. 20, No. 5, p. 529-532.

3. J. B. Henry (Ed.) Clinical Diagnosis & Management by Laboratory Methods (1991), Ch. 4: Statland & Winkel. p. 75.

4. N. W Tietz (Ed.) Textbook of Clinical Chemistry (1986), p. 480.

Ruth Jacobsen, MA, BS, CIS & CLPIb (NCA)

Mayo Medical Center, Rochester, MN