NEW YORK (Reuters Health) – A urinary tract infection may be more likely to spread to the bloodstream in patients who receive a red blood cell (RBC) transfusion, particularly in cases where “aged” blood is used, according to results of an observational study.

“The decision to order a RBC transfusion in a patient with a urinary tract infection should involve careful deliberation,” Mary A. Rogers, of the University of Michigan Ann Arbor and colleagues conclude in a Research Letter published September 26 in Archives of Internal Medicine.

They conducted a matched case-control study at the University of Michigan Health System, Ann Arbor from January 1, 2000 through September 30, 2008.

The study population comprised 299 adults with a positive urine and blood culture with the same microorganism during hospitalization (cases) and 670 adults with a positive urine culture who were at risk for a bloodstream infection but did not develop one (controls).

The researchers say the odds of developing a bloodstream infection increased nearly fivefold (adjusted odds ratio [OR], 4.84) in patients who received a RBC transfusion compared with those who didn’t.

And they saw a dose-response relationship, with the probability of developing a bloodstream infection increasing with the volume of RBCs transfused.

However, the relationship was nonlinear. The greatest unit increase in the probability of bloodstream infection occurred between 1 unit (250 mL) and 2 units (500 mL) of RBCs – the OR was 1.97 for 500 mL versus 250 mL – and then gradually increased with greater RBC volume.

The current results suggest that if a RBC transfusion is needed in a patient with a urinary tract-related infection, administering 1 unit of RBCs instead of 2 units of RBCs at a given time may confer less potential risk of bloodstream infection, the researchers say.

In addition, they found that prolonged RBC storage was associated with an increased odds of bloodstream infection, independent of the total volume of RBCs received; the odds increased by 63% for every week of increased storage time.

Dr. Rogers and colleagues point out in their paper that changes in the morphology and biochemistry of stored RBCs have been “extensively documented. Evidence regarding the effects of aged RBC transfusion is conflicting and therefore, there are trials under way to assess hypotheses related to the age of RBCs.”

Reuters Health asked Dr. John Hess, professor of pathology and medicine at the University of Maryland School of Medicine in Baltimore, to review the study. Transfusion medicine and red blood cell storage are two of his research interests.

“This study,” Dr. Hess said, “is in line with our evolving understanding of the risks of transfusion of longer-stored red cells where the recovered fraction goes down and so more hemoglobin and iron end up in the circulation where they can support bacterial growth.”

Nonetheless, he added, “it is still a retrospective study and finds an association but not proof of cause. Individuals who get more blood are sicker, and the more blood you get the greater are your chances for getting old blood. One more reason to think twice before giving a transfusion.”

Reference:

Arch Intern Med 2011;171:1587-1588.