Monday, 30 December 2019


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Written by Rita Losa Rodríguez | Gabriel Rodríguez Pérez | Guadalupe Ruiz Martín

Figure. Ammonium phosphate crystals "fern leaf" in urine. A) Optical microscopy, x400 increases. B) Polaryzed light filter, x200 increases.

We present the case of a young female patient, with a history of urinary tract infection with positive cultures, whose urine was sent from Primary Care for analysis. Its physical characteristics included dark brown urine and intense smell. Given the altered parameters of the test strip (pH 9, Proteins +, Nitrites + and Leukocytes +), we proceeded to make the sediment analysis. In this one, the presence of abundant crystalline forms of unusual morphology was highlighted, as shown in figure.
For its study and identification, bright-field optical microscopy was used. We observed a structure of the crystal nucleus very similar to the characteristic "coffin lid" shape of magnesium ammonium phosphates. In addition, they had an intense birefringence against polarized light (figure).

This atypical crystallization corresponds to the "fern leaf" variety of magnesium ammonium phosphates, characterized by extensions reminiscent of the plant morphology.

Its presence is unusual due to the specific conditions necessary for its formation, since it is almost always a urine contaminated by ureolitic bacteria that has delayed its analysis more than 8 hours.

The crystals of magnesium ammonium phosphate (Triple phosphate or struvite) appear mainly in alkaline urines (pH>8). They are colourless, pleomorphic and do not appear in healthy individuals. They are always associated with infections caused by ureolitic bacteria. Under determined physicochemical conditions ( large permanence time or high pH), these crystals precipitate into incomplete forms such as trapezoids, cruciform forms, prisms or into "fern leaf", as the referenced case.

Urinary infections by ureolitic bacteria are mainly caused by the genous Proteus, Morganella, Ureaplasma or Corynebacterium, with the possibility of chronification. They are generally associated with the elimination and/or formation of struvite calculi and carbonate apatite. Urease from microorganisms breaks down urea into carbon dioxide and ammonia. Ammonia is hydrolysed to ammonium and carbon dioxide to bicarbonate, alkalinizing the urine. This mobilises the urinary phosphates in order to compensate it, forming ammonium phosphate that ends up capturing magnesium and free urinary calcium.

The correct identification of the crystalline structures is fundamental for the diagnostic orientation; especially in this case, since the delay time between the sample collection and its analysis has affected its quality. We recommended to notify the doctor the appearance of these crystals due to incorrect preanalytical conditions, as well as the evaluation of a new request for urianalysis and / or urine culture.

The fact of that current auto-analysers are not capable of appropriately classifying these crystalline forms, together with the possible confusion in their identification with crystals of pharmacological origin, highlights the importance of observation under an optical microscope by properly trained personnel.

Ensuring pre-analytic conditions is critical in determining whether a finding is clinically relevant or an artefact.

  1. Castaño López MA, Díaz Portillo J, Paredes Salido F. La Patología a través del Laboratorio de Análisis Clínicos. Servicios de publicaciones de la Universidad de Cádiz. 1º ed, 2014.
  2. JiménezGarcía JA, Ruiz Martín G. El Laboratorio Clínico 2: Estudios de los elementos formes de la orina, Estandarización del Sedimento urinario. LABCAM 1º ed, 2010.
  3. Fogazzi Giovanni B. The urinary sediment. An integrated view.Elsevier. 3rd ed,2009.
  4. Romero Pérez P. Massive crystalluria associated with ureolitic urinary infection: An exceptional case.RevistaChilena de Urología.2017 Vol. 82 Núm. 2.


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