Inaccurate reporting of mineral composition by commercial stone analysis laboratories: implications for infection and metabolic stones

Abstract

Purpose: We determined the accuracy of stone composition analysis at commercial laboratories.

Materials and methods: A total of 25 human renal stones with infrared spectroscopy determined composition were fragmented into aliquots and studied with micro computerized tomography to ensure fragment similarity. Representative fragments of each stone were submitted to 5 commercial stone laboratories for blinded analysis.

Results: All laboratories agreed on the composition of 6 pure stones. Only 2 of 4 stones (50%) known to contain struvite were identified as struvite at all laboratories. Struvite was reported as a component by some laboratories for 4 stones previously determined not to contain struvite. Overall there was disagreement regarding struvite in 6 stones (24%). For 9 calcium oxalate stones all laboratories reported some mixture of calcium oxalate but the quantity of subtypes differed significantly among laboratories. In 6 apatite containing stones apatite was missed by the laboratories in 20% of samples. None of the laboratories identified atazanavir in a stone containing that antiviral drug. One laboratory reported protein in every sample while all others reported it in only 1. Nomenclature for apatite differed among laboratories with 1 reporting apatite as carbonate apatite and never hydroxyapatite, another never reporting carbonate apatite and always reporting hydroxyapatite, and a third reporting carbonate apatite as apatite with calcium carbonate.

Conclusions: Commercial laboratories reliably recognize pure calculi. However, variability in the reporting of mixed calculi suggests a problem with the accuracy of stone analysis results. There is also a lack of standard nomenclature used by laboratories.

Figure 1

Example of fragmented stone, showing the samples sent to the 5 laboratories. Images of samples shown on left, on mm grid paper, with representative micro CT slice on right. This stone was mixed uric acid and calcium oxalate monohydrate (COM), which are easily distinguished by micro CT, so that it is certain that each laboratory received a sample containing both materials. Bright white regions (arrowheads) on the micro CT images—seen within the COM—mark areas of apatite, which was present in minor amount in these samples.

Figure 2

Results of stone analysis from 5 laboratories (labeled A-E) for 9 calcium oxalate (CaOx) stones. Each row represents a stone that was fragmented into similar samples, one of which was sent to each laboratory. Labels to left indicate composition of stone as determined by micro CT of the original stone and by micro CT-guided dissection and infrared spectroscopy of a sixth sample. Each circle shows composition report by the laboratory; for example, the top left circle shows that Lab A reported the stone as containing 90% COD (CaOx dihydrate) and 10% COM. Nested circles (under Lab D) indicate that report gave details on inner and outer portions of specimen. AP: apatite.

Figure 3

Results of stone analysis for non-CaOx metabolic stones. Arrangement of data is as in Figure 2. UA: uric acid; BRU: brushite; AAU: ammonium acid urate. Labels to left indicate composition of stone as in Figure 2.

Figure 4

Results of stone analysis for stones related to infection. STR: struvite; NaUrate: sodium urate; AZ: atazanavir. CaOx in this figure combines reports of either COM or COD. Circles marked with a minus (-) show reports that miss struvite that was previously determined to be in the sample. Circles marked with a plus (+) show reports that listed struvite when none was detected in the original stone. Labels to left indicate composition of stone as in Figure 2.