Ulnar collateral circulation should be assured before a procedure is done, given the reported incidence of radial artery thrombosis or occlusion and the reported lack of ulnar collateral flow in 5% to 6% of patients [1]. The Allen test has been historically recommended for the bedside screening of ulnar flow [2]. Case reports of limb ischemia after radial artery cannulation despite a negative Allen test result have been attributed to emboli [3], not inadequate ulnar collateral circulation that was undetected.

The use of pulse oximetry has been recommended for enhancing the interpretation of results of this test and has been modified for critically ill or uncooperative patients. In pulse oximetry and the Allen test, the results are merely positive or negative and cannot be interpreted as more than that. Both tests identify patients whose low flow state or undetected ulnar flow with radial artery occlusion places them at risk. The Allen test, however, requires patient cooperation. These tests may erroneously exclude some patients who have subclinical flow that could be detected by other means.

Clinicians need to know whether subtler flow changes are significant, and they place patients at greater risk by doing radial artery puncture or cannulation. However, Williams and Schenken [4] reviewed 9020 cases in the literature and their own experience with more than 25 000 radical artery punctures. They found that these procedures were all done with no ischemic complications and concluded that the risk for complication is so low that prescreening with an Allen test is unwarranted. This conclusion would not be useful in the patient in the intensive care unit who is likely to be at higher risk for ischemia because of the use of vasoactive drugs and hypotension.

Cases of false-negative ulnar flow and false-positive ulnar flow should be identified. Glavin and Jones reported a sensitivity of 1.0 for oximetry compared with Doppler analysis of ulnar flow. Interestingly, they cited seven patients in their group in whom flow was detected by oximetry but not by Doppler. Unfortunately, they had no patients in whom oximetry failed to detect flow. This finding limits their conclusions. My colleagues and I have frequently identified patients in whom ulnar flow was compromised enough to warrant use of another site.

I agree with Dr. Kruse that diagnostic groups in which oximetry is not useful must be reported. We want to avoid the chauvinism of the Allen test, which may erroneously exclude too many cases from radial use, and the liberalism of "no testing," which may cause complications in the critically ill patient even if it does not cause complications in the general hospital population.