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Under CA law, the courts have held, that potential medical malpractice plaintiff must notify health care providers of his or her intent to sue at least 90 days before filing a complaint. (§ 364, subd. (a). See: Castenada v. Department of Corrections & Rehabilitation (2013) 212 Cal.App.4th 1051, 1068.) The purpose of section 364, also enacted as part of MICRA, "is to decrease the number of medical malpractice actions filed by establishing a procedure that encourages the parties to negotiate `outside the structure and atmosphere of the formal litigation process.'" (Woods v. Young (1991) 53 Cal.3d 315, 320 (Woods).) Section 364, subdivision (d), provides, "If the notice is served within 90 days of the expiration of the applicable statute of limitations, the time for the commencement of the action shall be extended 90 days from the service of the notice." Notwithstanding section 364, subdivision (d), states the time for filing shall be "extended," the Woods Court held the legislative purpose of section 364, subdivision (d), is "best effectuated by construing section 364(d) as tolling the one-year statute of limitations when section 364(a)'s ninety-day notice of intent to sue is served during, but not before, the last ninety days of the one-year limitations period. Because the statute of limitations is tolled for 90 days and not merely extended by 90 days from the date of service of the notice, this construction results in period of 1 year and 90 days in which to file the lawsuit." (Woods, at p. 325.).
So sending the letter only gives you 90 days from the date of the letter to file suit if you are going to file at all.