Evaluation of calcium urolithiasis in ambulatory patients. Comparison of results with those of inpatient evaluation

A protocol for ambulatory patients (ambulatory protocol or evaluation) was developed for the diagnosis of different causes of calcium urolithiasis; it consisted of a collection of two 24-hour urine samples with the subjects ingesting a random diet and another with the subjects adhering to an instructed diet restricted in calcium and sodium, followed by the test of "fast" and "oral calcium load." The results of ambulatory evaluation were compared with those of inpatient evaluation in 24 patients with various causes of calcium stones. The "match" in defining abnormalities between outpatient and inpatient evaluations was found to be virtually complete with respect to urinary calcium during fasting and following an oral calcium load, urinary oxalate and cyclic adenosine monophosphate (AMP), and serum calcium and immunoreactive parathyroid hormone (iPTH). However, six patients with normocalciuria during the inpatient setting (adhering to a low calcium diet) had hypercalciuria while adhering to the restricted diet used during outpatient evaluation. Serum phosphate and renal threshold phosphate concentrations were generally lower during the outpatient setting. The urinary uric acid level was significantly higher in outpatients, a finding probably reflecting the low purine content of the inpatient diet. The two evaluations yielded the same diagnoses in 18 patients: seven with absorptive hypercalciuria type I (hypercalciuria during both low and high calcium intakes), 2 with absorptive hypercalciuria type II (hypercalciuria only during a high calcium intake), one with hypophosphatemic absorptive hypercalciuria, three with renal hypercalciuria, one with primary hyperparathyroidism, two with hyperuricosuric calcium oxalate nephrolithiasis and two with enteric hyperoxaluria. Moreover, the ambulatory evaluation may be superior to the inpatient evaluation in disclosing hyperuricosuria. One patient in whom no obvious abnormalities were found during inpatient study, was found to have hyperuricosuric calcium oxalate nephrolithiasis during outpatient study. Hyperuricosuria was shown to coexist with other causes of calcium stones in 14 patients during outpatient evaluation, whereas it was present in three patients during inpatient study. However, utilizing the ambulatory evaluation, absorptive hypercalciuria type II was "confused" with the type I presentation in four patients, and hypophosphatemic absorptive hypercalciuria was "erroneously" diagnosed in one patient.