Devices and methods for detecting physiological target event such as events indicative of heart failure (HF) decompensation status are described. An ambulatory medical device (AMD) can detect device site maturation such as in a device encapsulation pocket, and classify the maturation status into one of two or more device site maturation states. The AMD can include an electrical impedance analyzer circuit that can measure a first maturation-insensitive impedance vector and a second maturation-sensitive impedance vector. At least one impedance vector can be selected or a composite impedance vector can be generated in accordance with the classified device site maturation state. The AMD can generate an impedance indicator using the selected or composite impedance vector, and detect a target physiologic event indicative of worsening of HF using the impedance indicator.