Biphasic anaphylactic reactions – How long should we observe?

A guest post by Anand Swaminathan (@EMSwami) of EM Lyceum and Essentials of EM fame.

ED Nurse: “You want to watch that allergic reaction for 6 hours? Seems like a long time.”
ED MD: “Yeah, I know but I want to make sure they don’t have a biphasic anaphylactic response.”
ED Nurse: “When does that usually happen?”
ED MD: “Well, uh . . . I’m not sure . . . hey did anyone see that 90 year old with dizziness yet? I should go see her now.”

If this conversation seems familiar, you’re not alone. We’ve all been taught that patients with allergic reactions have a risk of recurrent reaction. What we fear, is an anaphylactic episode that improves, the patient goes home and then has recurrent anaphylaxis and dies. As a result, many of us have been taught that if the patient resolves, you watch them for 4-6 hours and if they remain okay, they can go home.

However, when you look at the literature, it shows that the biphasic reaction can be delayed. It can occur anywhere from 5 minutes up to 3-4 days out. So what is the optimal duration of observation?

Enter this retrospective, chart review from the University of British Columbia. The researchers found 2,819 patients with allergic reactions at two institutions (496 classified as anaphylaxis) and the scoured databases for representations within 7 days. They further separated out those patients with clinically important biphasic reactions. Overall, there were five patients (0.18%) who had clinically important biphasic anaphylactic responses in seven days. Of these, two had immediate biphasic anaphylaxis during their initial Emergency Department visit and three experienced a delayed response up to six days out of their initial visit. Two of the biphasic reactions were in patients initially presenting with anaphylaxis (0.40%) and three were in patients presenting with simple allergic reactions (0.13%).  There were no deaths identified.

The study has the typical limitations of any retrospective chart review. One major concern is that patients presented to EDs other than the original two sites. The researchers did, however, check the regional database for representations and checked the provincial database for mortality.

Although the study isn’t perfect, it suggests that the biphasic response may be lower than previously thought. Prior studies have shown biphasic reaction rates ranging from 3 – 20% (Tole 2007). As a result, some authors have recommended observation for up to 24 hours after an anaphylactic reaction. The truth is that there are no consistent recommendations about observation. The authors conclude that,

“although extended observation would be justified in patients with severe or protracted anaphylaxis, the added costs and resource use involved in routine prolonged monitoring of patients whose symptoms have resolved may worsen ED crowding while likely adding little to individual patient safety.”

While it’s hard to recommend changing practice based on a retrospective study, this is the largest study done on the subject and offers very reassuring numbers. The bottom line is that clinically important biphasic reactions are rare (less than 1%) and can occur days after the initial reaction. There is no 100% safe observation period. After symptom relief, and decision for discharge, patients should be given epinephrine auto injectors and taught how to use them. Any potential inciting allergens should be removed and follow up should be arranged with an allergist. With a clinically important biphasic response rate < 0.5%, extended observation seems to be unnecessary.
References
Grunau BE et al. Incidence of Clinically Important Biphasic Reactions in Emergency Department Patients with Allergic Reactions or Anaphylaxis. Ann of EM 2013. ePub http://dx.doi.org/10.1016/j.annemergmed.2013.10.017

Tole JW, Lieberman P. Biphasic Anaphylaxis: Review of Incidence, Clinical Predictors and Observation Recommendations. Immunol Allergy Clin N Am 2007; 27: 309-26.
http://www.ncbi.nlm.nih.gov/pubmed/17493505