Comprehensive metabolic evaluation using 24-hour urine testing is essential for those at increased risk of stone formation (see Statement 5) and appropriate for motivated patients after their first stone, as it identifies modifiable urinary abnormalities that guide targeted therapy. Because diurnal variation in calcium, oxalate, citrate, uric acid, and pH is common, 24-hour urine collections comprise the standard approach by which urinary metabolites are measured. Collections should be performed when individuals are in their usual state of health consuming their usual diet. While the Panel recommends that urine collection on a typical diet provides the optimal conditions to reflect normal intake, it is recognized that there are circumstances in which clinicians may choose to have a patient continue a certain supplement or consume certain foods in order to determine if the supplement/food is impacting their stone risk. Controversy persists over the optimal number of baseline urine collections (1 or 2) required to provide the best assessment of underlying abnormalities, maximize analytic reliability, and capture dietary and physiologic variability without undue burden on the patient. The Panel recognizes that data supports both strategies.117 Each collection requires discarding the first void, collecting all urine for exactly 24 hours, and using appropriate preservatives for processing unless otherwise specified by the laboratory. Urine creatinine can be utilized as a marker for completeness of collection. While multiple collections might provide greater accuracy and reproducibility, this approach must be balanced against patient convenience and adherence with obtaining multiple collections. The recommended metabolic testing panel, when available, includes urinary volume, pH, calcium, oxalate, uric acid, citrate, sodium, potassium, creatinine, ammonium (NH₄⁺), and either sulfate or urea nitrogen, with optional analytes such as magnesium, phosphate, chloride, and cystine screening when clinically indicated (Table 4). Cystine screening should be performed in children with kidney stones and patients with a strong family history of cystinuria. This comprehensive panel enables estimation of net acid excretion, stone inhibition, and supersaturation which are thought to predict risk of stone recurrence. Commercial laboratories provide one-stop comprehensive testing, although not all clinicians have access to such services. Supersaturation indices are increasingly provided by commercial laboratories using validated algorithms such as EQUIL2, which integrate multiple analytes into stone-specific risk metrics to enhance understanding of pathophysiologic drivers of recurrence.118 Serum studies including serum calcium, bicarbonate, phosphorus, uric acid, 25-hydroxy vitamin D, and 1,25-dihydroxy vitamin D can further facilitate diagnosis of primary hyperparathyroidism, distal RTA, gout, or vitamin D-related hypercalciuria. Each of these metabolic etiologies for stone disease is treatable, thus providing a rationale for serum testing. After dietary or pharmacologic interventions, repeat 24-hour urine testing at approximately 3-6 months after initiation of intervention allows for verification of therapeutic effect and optimization of management, with annual follow-up testing recommended in persistent high-risk populations. Frequency of testing depends on the type of intervention initiated (i.e., dietary versus medical therapy, entailing less versus more frequent follow-up). Once patients are stable on therapy with no evidence of stone recurrence or growth, 24-hour urine testing can be reduced in frequency or omitted. Intensity of follow-up should be tailored to the stability of the patient.