Even with testing, toxicity can happen unfortunately. My husband was tested very regularly but because he has multiple health issues he ended up going into kidney failure anyway, was put on dialysis and in the ICU for a week (completely psychotic because the dialysis pulled all meds off his system abruptly). Rather nightmarish experience that swore me off lithium for myself even though I had never had problems with it. What happened to him is considered rare, but it came within hours of being deadly. Just not worth the risk for us. Never again.