To the Editor:
We read with interest the letter of Mulroy and Glosten  and agree with the sentiment that "when catheter aspiration is negative, it means nothing." We would also like to address their question of whether there is something better to use in this patient population.
We reported the results of a test in a series of 278 patients . Air (0.5-1 mL) was injected through an epidural catheter, followed by 1 mL of saline. First, the catheter is held up: a rapid decrease in the saline level in the catheter indicates placement of the tip in a low-pressure space. Thereafter, the catheter is held below the level of the insertion point. Outward movement of the saline meniscus, which slows and stops, indicates lack of placement in the subarachnoid space. The return of air bubbles in the catheter indicates lack of placement in the subarachnoid space and lack of placement in a blood vessel. There is "proof" of backward flow.
We believe that this nonpharmacological test is a valuable addition to the tests available. The three-step test addresses the twin problems of the lack of reliability of catheter aspiration for either blood or cerebrospinal fluid and "when catheter aspiration is negative, it means nothing" . The three-step test gives a clear indication of successful back flow from the epidural catheter-with such an indication of back flow, the clinician would have more confidence in accepting lack of cerebrospinal fluid and lack of blood as placement of the epidural catheter outside the subarachnoid space and outside a blood vessel.
We agree with the authors that comparison of the reliability of the available tests is needed and suggest that the three-step test be considered.
W. Bosseau Murray, MB ChB, FFARCS (Lon), MD
Department of Anesthesia; Pennsylvania State University College of Medicine; Hershey, PA
Andrzej Trojanowski, Lek Med, FFARCS, DEAA, DA
Department of Anesthesia; Houlton Regional Hospital; Houlton, M
Drs. Mulroy and Glosten did not wish to respond.
1. Mulroy M, Glosten B. The epinephrine test dose in obstetrics: note the limitations. Anesth Analg 1998;86:923-5.
2. Trojanowski AT, Murray WB. Avoiding epidural complications: a test to detect correct placement. S Afr Med J 1995;85:531-4.