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Dextrose-free local anesthetics and opioids, alone and in combinations, are being used increasingly to provide subarachnoid anesthesia and analgesia. These dextrose-free drugs have been described as hypobaric by some and isobaric by others. To accurately classify anesthetics with regard to baricity, the density of cerebrospinal fluid (CSF) must be known. The authors sought to determine the exact density of human CSF, and determine whether CSF density is altered by pregnancy.Density measurements accurate to 0.00001 g/ml were made at 37.00 degrees C, using a mechanical oscillation resonance frequency density meter. Cerebrospinal fluid samples were obtained from 44 patients during spinal anesthesia. Five groups were studied: men, and premenopausal, postmenopausal, term pregnant, and postpartum women.Mean CSF densities in men (1.00064+/-0.00012 g/ml), postmenopausal women (1.00070+/-0.00018 g/ml), and nonpregnant premenopausal women (1.00049+/-0.00004 g/ml) were significantly greater than in term pregnant (1.00030+/-0.00004 g/ml) and postpartum (1.00034+/-0.00005 g/ml) women. Cerebrospinal fluid density did not correlate with age.Mean CSF density varies in different patient subpopulations. Pregnancy and the immediate postpartum period are associated with the lowest CSF densities. In addition, the cutoff values defining hypobaricity (mean CSF density minus three standard deviations) are greater than previously reported. Accurate CSF density values should be used when considering baricity as a mechanism for clinical observations of dextrose-free intrathecal local anesthetics and opioids. Gestational status also should be considered.