Explain why lability of affect and reactivity of mood can be easy to confuse in a brief clinical encounter, despite being different findings.

Both findings involve a patient's emotional state changing more than once during an interview, so a brief encounter can make them look superficially similar — in each case the clinician observes a person whose mood is not simply flat and unchanging. What actually separates them is the relationship between the change and its trigger. Reactivity of mood tracks real external circumstances proportionately: the mood moves because something genuinely mood-relevant happened, and by roughly the amount that event would be expected to produce. Lability of affect involves shifts that are abrupt and excessive relative to whatever prompted them, if anything clearly did. Only by weighing each emotional shift against its actual trigger, rather than simply counting how many shifts occurred, can the two be reliably told apart.