The hardness test is key to diagnosis of factitious calcium carbonate stones

We read with interest the review by Kinns et al. on the role of the laboratory in Munchausen Syndrome. ¹ In their excellent and thought-provoking review of the wide range of ways in which the laboratory can be involved, there was limited space for the specialist area of stone analysis. We wish to add a few key points for the benefit of laboratories that analyse renal or other calculi.

In our experience as a referral centre for the Republic of Ireland the single most important diagnostic criterion for identifying a stone as factitious is the degree of hardness in relation to the chemical composition. By way of illustration we outline below two sets of cases; one from the wet-chemistry era (1989) and one after our introduction of infrared spectroscopy (2012–2013).

Case A (human stone) and Case B (non-human stone) had very similar characteristics on wet chemistry analysis – size (2 mm and 3 mm), shape (irregular), combustion (inorganic), urate (negative), cystine (negative), ammonia (negative), carbonate (++), and calcium (approx 35% and 40%, respectively). The big difference was the hardness, the human stone being classified as ‘soft’ and the factitious one as ‘very hard’.

Case C (human stone) and Cases D and E (non-human stones) also had very similar characteristics on infrared spectroscopy analysis – all three were classified as calcium carbonate by infrared spectroscopy using both a commercial spectrum library (Advanced KBrOC Library) and our in-house Mater Patient ATR Library ² with matches greater than 90% for ‘calcite’ or ‘calcite + vaterite’. Hardness was judged as ‘soft’ for Case C, ‘hard’ for D, and ‘very hard’ for E.

Calcium carbonate stones of human origin are an uncommon finding worldwide. The most common non-human stones in our experience are those picked up from the ground and either declared as passed during urination or physically inserted into the urethra. Our experience shows that such stones are invariably calcium carbonate and distinguishing them from stones produced within the body is very important for clinical management of the patient. Modern stone analysis methods aid in the determination of stone constituents but it is the very simple ancient step of crushing the stone with mortar and pestle that is key to establishing whether a calcite stone was produced in vivo or picked off the ground.

Kinns et al. rightly make the point that a factitious stone is not necessarily Munchausen Syndrome. Case E above undoubtedly was, and after initial referral to a urologist our analysis resulted in a review of her management. Case D, however, was a known stone former so his result was unexpected. It transpired that the man had passed a stone while out in the fields and picked up from the ground what he thought was his stone to produce as evidence to the doctor.

While Kinns et al. state that multiple spurious stone submissions by the same patient do warrant further investigation, we contend that such investigation should start right from the first finding of a factitious stone. Investigations should be undertaken, involving both clinical and diagnostic departments in the hospital, with a view to determining which category the incident fits into – Munchausen, accidental (e.g. Case D), or opportunistic (e.g. some drug addicts may use such methods to obtain a prescription for morphine to alleviate the severe pain from a ‘kidney stone’).

Laboratory investigation of stones is often the starting point for diagnosis of Munchausen Syndrome in selected patients presenting with factitious stones and this service is also available as backup to the psychiatric service. In a small number of cases we have even used radiometric dating of stones by a geology laboratory to prove (by age of stone) the factitious nature of a stone presentation; an expensive test in itself but one which proved clinically decisive on each occasion.