Investigation of renal stones
All patients presenting with acute renal stones should have a measurement made of their renal function and plasma calcium and urate. The importance of knowing their renal function is to determine the urgency with which further investigations and treatment should be performed so that further renal damage can be avoided. Other investigations will include imaging of the renal tract and urine culture (including examination for fastidious organisms). This diagnostic algorithm is designed for the investigation of patients following their acute presentation.
Renal stones will occur in 8-15% of Europeans and North Americans during their lives. The majority of stones (~80%) are composed of calcium oxalate and the remainder are composed of uric acid (5-10%), struvite or carbonate apatite (secondary to infection), cystine (~1%) and rare stones.
Investigation of the aetiology of renal stones is based on urine chemistry rather than analysis of the stone itself because the accuracy of stone analysis is generally poor and, even when accurate, does not explain the pathophysiology of stone formation except in the rare inherited disorders eg cystinuria.
The rationale of not investigating those patients with a single stone and no risk factors is largely arbitrary since approximately 75% of patients presenting with a renal stone will have a recurrence during the next 20 years. The decision about the degree of investigation and the potential for life long therapies will depend on the patient.
The presence of diseases of the chest and GI tract and some drugs are important insofar as they affect the excretion of acid in the urine (see below).
The importance of measuring urine pH
Urine pH should be measured on fresh urine samples as urine is not strongly buffered and CO2 will evaporate. The inability to acidify urine during an episode of acidosis (either spontaneous or therapeutically induced) indicates one of the forms of renal tubular acidosis and a direct tubular effect on calcium excretion. However, the main reason for measuring urine pH, since few patients with renal stones will have RTA, is to determine if stone formation is secondary to subtle changes in urinary pH. This may reduce the solubility of supersaturated salts in urine and enhance crystal formation. In view of this we would suggest that normal urine has a pH range of 6-7 and that it may be appropriate to consider therapies to alter urine pH in recurrent stone formers whose urine pH is without these limits.
Urinary urea is measured to give a guide to protein intake for the evaluation of urate (ie purine) excretion.
24 hours urine volumes
It is important to record measurement of the total urine volume passed over each of the 24 hour collections since poor urine flow effectively increases the concentration of salts and will enhance crystal formation.
Hypertension is considered a risk factor since there is a strong association between the two conditions with a suggestion that hypertension develops after the presentation of stones. Moreover, hypertensive patients have a considerably higher degree of calciuria than controls.
Renal stones in children
This algorithm is not suitable for children with renal stones
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