For a tooth with open apex, treated with calcium hydroxide for apexification, the most reliable sign before obturation is:

Weine recommends that the apexification procedure be completed in two appointments. After instrumentation, irrigation and drying of the canal during the first appointment, he advises sealing a sterile, dry, cotton pellet in the pulp chamber for 1 to 2 weeks. Placing a calcium hydroxide dressing in the canal is optional at the first appointment.  During the second appointment, the debridement procedures are repeated before the canal is filled with a thick paste of calcium hydroxide and CMCP or calcium hydroxide in a methylcellulose paste. Whether the tooth is filled in one or two appointments (or more) should be determined to a large extent by the clinical signs and symptoms present and to a lesser extent by operator convenience. All signs and symptoms of active infection should be eliminated before the canal is filled with the treatment paste.  Absence of tenderness to percussion is an especially good sign before the canal is obturated. Because of the wide-open access to periapical tissues, it is not always possible to maintain complete dryness in the root canal.  If the canal continues to weep but other signs of infection seem to be controlled after two or three appointments, the dentist may elect to proceed with the calcium hydroxide paste treatment.