Variation in plasma and urinary inorganic phosphate and pyrophosphate in normal subjects and in patients with acromegaly or osteoarthritis.

Plasma inorganic pyrophosphate (PPi) and inorganic orthophosphate (Pi) levels, urinary PPi and Pi excretion were measured every 2 h for a day in 4 normal subjects on a normal diet and again while fasting; 5 patients with osteoarthritis and 4 patients with acromegaly were also studied. Plasma Pi values were elevated at night in all subjects and were highest in the acromegalic patients. Mean plasma PPi was lower during fasting and correlated with Pi levels in all non-fasting subjects. Urinary Pi and PPi were strongly correlated; both were lower in fasting subjects. Food intake and the time of blood sampling are important biological variables that must be considered when interpreting plasma PPi levels.