Background: Cerebrospinal fluid (CSF) leak of the anterior skull base is an abnormal communication between the subarachnoid space and the nasal cavity or the paranasal sinus. Its presence increases the incidence of complications, as meningitis, with risk of sequelae. The early and correct diagnosis of the CSF leaks is important to perform surgical treatment as soon as possible. The CSF detection in nasal fluids could be made through the beta2-transferrin (beta2Tr) immunoelectrophoretic test, an expensive and cumbersome immunologic test, not available to the majority of the Brazilian hospitals. Recently, the detection of beta-trace protein (betaTP) for identification of CSF leaks has been described. The literature suggests there may be similar sensitivity and specificity to tests of beta2Tr, because betaTP is also a specific brain protein and it is present in CSF in high concentrations. The majority of Brazilian hospitals have the nephelometric equipment for betaTP detection. This study was designed to determine threshold values for betaTP concentrations in nasal cavity secretions as a predictor of CFS leak. Methods: Nasal secretions were collected from patients with suspicion of CSF leak and from healthy volunteers. Pure CSF samples were used. A nephelometric assay was used to determine the betaTP concentration in samples. Results: Values between 0.244 and 0.496 mg/L were determined for the cutoff. Conclusion: Beta-trace concentrations above 0.496 mg/L are highly suggestive of the presence of CSF in examined nasal secretion. Positive predictive value and negative predictive value were 100%. BetaTP nephelometric test can predict the side of the CSF leak. Copyright © 2009, OceanSide Publications, Inc.236585590Har-El, G., What is "spontaneous" cerebrospinal fluid rhinorrhea? 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