The kidney is involved in a number of physiological processes, including fluid and electrolyte balance. Kidney function has been associated with obstructive airways disease, however the nature of this relationship has been difficult to unpick. Specifically, it is unclear whether kidney function affects airways disease, vice versa, or whether the association is instead attributable to confounding factors. Insight into the nature of this relationship is of clinical relevance, as the identification of renal impairment as a causal risk factor for obstructive lung disease would offer mechanistic insight and therapeutic opportunities. However, conventional observational studies exploring this have been unable to infer causation, due to the possibility of unmeasured or unknown environmental confounding and reverse causation.