Objectives Hereditary xanthinuria is a rare, autosomal and recessive disorder characterized by severe hypouricemia and increased xanthine excretion, caused by a deficiency of xanthine dehydrogenase/oxidase (XDH/XO, EC: 1.17.1.4/1.17.3.2) in type I, or by a deficiency of XDH/XO and aldehyde oxidase (AOX, EC: 1.2.3.1) in type II. Case presentation We describe a novel point mutation in the XDH gene in homozygosis found in a patient with very low serum and urine levels of uric acid, together with xanthinuria. He was asymptomatic but renal calculi were discovered during imaging. Additional cases were found in his family and dietary recommendations were made in order to prevent further complications. Conclusions Hereditary xanthinuria is an underdiagnosed pathology, often found in a routine analysis that shows hypouricemia. It is important for Laboratory Medicine to acknowledge how to guide clinicians in the diagnosis.

Resumen Objetivos La xantinuria es una enfermedad rara, de herencia autosómica recesiva caracterizada por la presencia de hipouricemia y elevada excreción de xantina, provocada por el déficit de xantina deshidrogenasa/oxidasa (XDH/XO, EC: 1.17.1.4/1.17.3.2) en el tipo I, o por el déficit de XDH/XO y aldehído oxidasa (AOX, EC: 1.2.3.1) en el tipo II. Métodos Describimos una nueva mutación puntual en homocigosis en el gen XDH en un paciente con niveles muy bajos de ácido úrico en suero y orina y xantinuria. Aunque el paciente se encontraba asintomático, se objetivaron cálculos renales en las pruebas de imagen. Resultados Se detectaron otros casos en su familia, y se le hicieron recomendaciones dietéticas para prevenir futuras complicaciones. Conclusiones La xantinuria hereditaria es una patología infradiagnosticada, que se suele descubrir accidentalmente al detectar en un análisis rutinario la presencia de hipouricemia. Es importante que la medicina de laboratorio sepa orientar a los facultativos en su diagnóstico.

Objectives We describe a woman with constantly elevated hCG levels in serum. Since assay interference, pregnancy or cancer did not explain the elevated levels, we measured the concentrations of hCG, its β subunit (hCGβ) and its core fragment (hCGβcf) in serum and urine using specific assays, to understand the nature of the elevated hCG levels. Methods We used 3 assays for total hCG (these assays also recognize hCGβ and to various degrees hCGβcf), 3 for intact hCG heterodimer, 3 for free hCGβ and one for hCGβcf. Results With an hCG assay detecting total hCG the serum concentrations were in the range of 150–260 IU/L for the whole study period of almost 5 years, except for a peak of 1,200 IU/L, coinciding with a spontaneous abortion. Quantitation of different forms of hCG with specific immunoassays showed that the immunoreactivity in serum consisted of hCGβ. Urine contained hCGβ and hCGβcf. Conclusions The laboratory findings are in keeping with familial hCG syndrome. However, so far the condition remains to be determined in any family members. Elevated hCG levels without any explanation are problematic as they cause suspicion of cancer or ectopic pregnancy and may lead to harmful therapy. Specific assays, as used here, will aid in diagnosis of such cases.