A dangerous condition gets missed, treatment is delayed, and a problem that might have been manageable turns into a medical emergency. That is the basic idea behind failure to diagnose: a healthcare provider does not identify a disease, injury, infection, or other condition when a reasonably careful provider should have done so under similar circumstances.

In practice, it is not enough that a diagnosis was hard or that a doctor turned out to be wrong. A claim usually depends on showing a departure from the accepted standard of care and that the delay or missed diagnosis caused real harm. If the outcome would have been the same even with an earlier diagnosis, causation becomes much harder to prove. These cases often turn on medical records, test results, symptoms that were overlooked, and expert opinions about what should have happened next.

For a New York malpractice claim, this issue matters a great deal because the case usually needs expert support at the outset. Under New York CPLR 3012-a, a plaintiff's lawyer generally must file a certificate of merit stating that a licensed physician has reviewed the facts and believes there is a reasonable basis for the action. New York also has a special rule for certain cancer cases: Lavern's Law (2018) extended the discovery rule for some acts involving failure to diagnose cancer or a malignant tumor, allowing the filing deadline to run from discovery rather than only from the missed diagnosis itself.