What is the difference between reliability and validity of diagnosis?

Diagnosis involves identifying a disease by its signs, symptoms and from the results of various diagnostic procedures. Reliability can be defined as consistency between two measurements. There are two main types of reliability when it comes to diagnosis: inter-rater reliability, which is high when different psychiatrists agree on a patient's diagnosis when using the same diagnostic system, and test retest reliability, which involves a patient being diagnosed with the same disorder by the same practitioner at two different times. Validity refers to the extent to which a diagnosis is accurate. The diagnosis should lead to appropriate treatment and prognosis. Validity presupposes reliability; a diagnosis cannot be valid if it is not reliable. It can however be reliable without being valid.