Quantitative test to distinguish spinal fluid from saline solution in combined spinal-epidural anesthesia

OBJECTIVES: To validate the use of a digital blood glucose meter for detecting the presence of spinal fluid during combined spinal-epidural anesthesia in terms of specificity, positive and negative predictive values, and likelihood ratios. PATIENTS AND METHODS: Validation was studied in 30 patients scheduled for surgery under combined spinal-epidural anesthesia. A positive finding, defined as detection of spinal fluid return or aspiration by the epidural or spinal needle, was compared with results of standard reference tests (the pattern of sensory or motor block after administration of the local anesthetic). After locating the epidural space with saline solution, the test was performed and 3 mL of local anesthetic was administered. If no sensory or motor blockade was evident, the test was considered a true negative. Spinal puncture was then performed, the test was repeated, and 2 to 3 mL of local anesthetic was injected. The test was considered a true positive if sensory or motor blockade was evident. These findings entered into the validation analyses. RESULTS: Sensitivity was 100%, specificity 94%, positive predictive value 93%, negative predictive value 100%, the positive likelihood ratio 15.5, and negative likelihood 0. CONCLUSION: Blood glucose meter readings provide a valid quantitative measure for distinguishing spinal fluid from saline solution during combined spinal-epidural anesthesia. The method, which uses a readily available device, is easy to use to rule out the presence of spinal fluid.