Abstract: SA-PO0546
A Case of Severe Hypophosphatemia with Renal Phosphate Wasting as a Manifestation of Hemophagocytic Lymphohistiocytosis (HLH): Role of Fibroblast Growth Factor 23 (FGF-23)
Session Information
- Fluid, Electrolyte, and Acid-Base Disorders: Clinical - 3
November 08, 2025 | Location: Exhibit Hall, Convention Center
Abstract Time: 10:00 AM - 12:00 PM
Category: Fluid, Electrolytes, and Acid-Base Disorders
- 1102 Fluid, Electrolyte, and Acid-Base Disorders: Clinical
Authors
- Cacciapuoti, Martina, Stanford Medicine, Stanford, California, United States
- Bahrainwala, Jehan Z., Stanford Medicine, Stanford, California, United States
- Bhalla, Vivek, Stanford Medicine, Stanford, California, United States
- Abra, Graham E., Stanford Medicine, Stanford, California, United States
Introduction
In acute lymphocytic leukemia (ALL), HLH may be the presenting sign or a complication of subsequent CAR-T. Diagnostic criteria include elevated ferritin. HLH through inflammation may recapitulate iron deficiency, increasing FGF23 and hence PO4 excretion in the setting of genetic or acquired defects in FGF23 cleavage. However, in post-CAR-T hypophosphatemia, elevated intact FGF23 has not been reported, and thus the mechanism is not fully understood. We present a case of severe hypophosphatemia with HLH and elevated FGF23.
Case Description
A 20 y/o female presented with fatigue, fevers and epistaxis and was diagnosed with HLH and ALL. Labs showed severe hypophosphatemia (0.9 mg/dL). She had no personal or family history of hypophosphatemia or fractures. Chest CT showed diffuse myocardial calcifications, but X-rays excluded fractures or osteomalacia. She had normal to low serum PTH and low 1,25 vitamin D (9 pg/mL). ALL treatment consisted of immunotherapy with resolution of HLH and hypophosphatemia. Subsequently, she received CAR-T (obecabtagene autoleucel). Thirty-nine days later, she was in remission from ALL but developed recurrent HLH that persisted despite cytokine inhibitory therapy. She again developed hypophosphatemia despite PO4 and Vitamin D supplementation, and nephrology was consulted. She exhibited renal PO4 wasting and urinanalysis was inconsistent with Fanconi syndrome. Intact (1294 pg/mL; normal range ≤59) and c-terminal FGF23 (1510 RU/mL; normal range 44-215) were elevated (ratio 0.86). Genetic testing excluded known hereditary syndromes of elevated intact FGF23, and the patient had not received ferric carboxymaltose. Ferritin showed a significant negative correlation with serum PO4 (r-0.73, p < 2.2e-16) despite aggressive PO4 supplementation.
Discussion
We speculate that the hyperinflammation of HLH was necessary and sufficient to explain high levels of intact FGF23 and PO4 wasting in the absence of a genetic or acquired defect in FGF23 cleavage. The inverse correlation between ferritin and serum PO4 supports this hypothesis. For the first time, we propose FGF23 as a potential driver of hypophosphatemia after CAR-T, specifically in the setting of HLH. In conclusion, we suggest FGF23 evaluation in this setting in patients unresponsive to standard supplementation.