Background: Reduced renal function is associated with a variety of biochemical abnormalities. However, the extent of these changes and their magnitude in relation to renal function is not well defined, especially among individuals with mild to moderate chronic renal insufficiency (CRI).

Methods: We analysed the Third National Health and Nutrition Examination Survey (NHANES III; 1988-1994) data for 14722 adults aged >/=17 years with measurements of serum creatinine and all electrolytes including ionized calcium. General linear models were used to determine the relationship between mean concentrations of electrolytes and different levels of Cockcroft-Gault creatinine clearance (CrCl). Sample weights were used to produce weighted regression parameters.

Results: Changes in mean serum phosphorus and potassium concentration were evident at relatively modest reductions in CrCl (around 50 to 60 ml/min). Changes in the anion gap and mean levels of ionized calcium and bicarbonate were not apparent until CRI was advanced (CrCl </=20 ml/min). For example, compared with women with CrCl >80 ml/min, those with CrCl 60-50, 50-40, 40-30, 30-20 and </=20 ml/min had mean serum phosphorus concentrations that were higher by 0.1, 0.1, 0.2, 0.3 and 0.8 mg/dl (all P<0.05), and mean serum potassium concentrations that were higher by 0.1, 0.1, 0.1, 0.2 and 0.4 mmol/l (all P<0.05), respectively. These changes were independent of dietary intake and the use of angiotensin converting enzyme (ACE) inhibitors or non-steroidal anti-inflammatory drugs (NSAIDs).

Conclusions: Increases in serum phosphorus and potassium levels are apparent even among people with mild to moderate CRI. These findings should be broadly generalizable to the larger CRI population in the United States. Subtle elevations in serum phosphorus might contribute to the initiation and maintenance of secondary hyperparathyroidism, which is known to occur in mild to moderate CRI.