Hypophosphatemia Associated with Ferric Carboxymaltose (FCM)

Incidence and Severity

Ferric carboxymaltose induces hypophosphatemia in 47‑75 % of recipients, markedly higher than the <10 % incidence observed with other intravenous iron products. 1
Hypophosphatemia severity is categorized as: mild < 2.5 mg/dL, moderate 2.5‑2.0 mg/dL, severe 2.0‑1.0 mg/dL, and potentially fatal < 1.0 mg/dL. 1
In the PHOSPHARE‑IBD trial, 51 % of participants receiving FCM developed serum phosphate < 2 mg/dL at any point from infusion to day 35. 1
Severe hypophosphatemia may persist for up to 6 months after a single FCM dose, although the exact duration is uncertain. 1

FCM triggers a pronounced rise in intact fibroblast growth factor‑23 (iFGF23), which drives renal phosphate wasting and consequent hypophosphatemia. 1
The resulting phosphate loss initiates a cascade of secondary electrolyte disturbances (e.g., secondary hyperparathyroidism and reduced active vitamin D levels). 1

Moderate to severe hypophosphatemia commonly produces fatigue, proximal muscle weakness, generalized asthenia, and in extreme cases, respiratory insufficiency. 1
These symptoms can closely mimic iron‑deficiency anemia, creating a notable diagnostic pitfall. 1

Repeated FCM infusions should be avoided in patients at high risk for bone complications, as cumulative exposure can lead to osteomalacia and fractures. 1
Groups at elevated risk include individuals with:
(All listed as a single cited block.) 1
Patients with impaired renal function have a reduced likelihood of developing hypophosphatemia; incidence in heart‑failure cohorts with renal compromise was 8.1 % versus 30‑55 % in other populations. 1

Prolonged or severe hypophosphatemia associated with high cumulative FCM doses can cause hypophosphatemic osteomalacia. 1
Repeated FCM courses have been linked to an increased incidence of fractures. 1

For patients identified as high risk, an alternative intravenous iron formulation should be considered whenever feasible. 1
When FCM use is unavoidable in at‑risk individuals, serum phosphate should be measured before the initial dose and prior to each subsequent dose, especially during repeated or high‑dose regimens. 1

Other IV iron products demonstrate a markedly lower hypophosphatemia incidence (<10 %). 1
Preferred alternatives for patients requiring repeated iron therapy include low‑molecular‑weight iron dextran, ferumoxytol, and ferric derisomaltose. 1
Selecting these alternatives is strongly advised to mitigate the risk of osteomalacia and fractures in patients needing multiple iron infusions. 1