The lab, which handles evidence for criminal cases, is currently processing more than 15,700 cases. SEATTLE — Washington State Patrol officials say a backlog in toxicology testing is delaying impaired driving investigations across the state as forensic scientists work overtime to keep up. The lab, which handles evidence for criminal cases, is currently processing more than 15,700 cases, far exceeding its goal of delivering results within 60 to 90 days. The backlog affects a wide range of cases, though about 65 percent of the lab’s work is tied to impairment investigations. According to State Patrol spokesperson Chris Loftis, delays can…