During or after a lumbar puncture the question frequently arises whether blood has been introduced into the spinal fluid by the spinal needle, resulting in a traumatic tap. There are several useful differential points. Xanthochromia, if present, suggests previous bleeding. However, a nonxanthochromic supernatant fluid does not rule out the diagnosis, since xanthochromia may be absent even when subarachnoid bleeding has occurred many hours before. A second differential point utilizes the fact that the standard method for collecting CSF involves catching the specimen in three consecutively numbered tubes. If blood was introduced by a traumatic tap, more blood should appear in the first tube, less in the second, and even less in the third, as the bleeding decreases. Previous CSF bleeding should distribute the RBCs equally throughout the spinal fluid and characteristically shows approximately equal numbers of RBCs in each of the three tubes. Therefore, RBC counts can, if necessary, be requested for all three tubes. However, sometimes traumatic taps, if severe, can yield roughly equal numbers of RBCs in each tube. As noted previously, blood in the CSF may falsely alter the various chemical tests.