Describe parenteral iron therapy — its aetiology, clinical features and management.

Definition

Parenteral iron therapy is the administration of iron by the intravenous or intramuscular route to treat iron-deficiency anaemia when the oral route is unsuitable. It guarantees that the calculated iron deficit is delivered and the stores replenished.

Indications

• Intolerance of, or non-compliance with, oral iron.
• Contraindication to oral iron (malabsorption, severe gastrointestinal disease).
• Late presentation (after ~30 weeks) with moderate-to-severe anaemia where there is too little time for oral therapy to work.
• Severe anaemia needing rapid correction but not requiring transfusion.

Preparations and Dose

• Iron sucrose (20 mg elemental iron/mL) — 100–200 mg per sitting IV, on alternate days; safe with few side-effects (preferred).
• Sodium ferric gluconate — 12.5 mg/mL, given in divided doses.
• Iron dextran — allows a total-dose infusion (the whole calculated deficit in one sitting) but carries a greater risk of anaphylaxis, so a test dose is required.

The total iron required is calculated from the haemoglobin deficit plus an allowance (≈ 500–1000 mg) for stores. A commonly used formula is: iron deficit (mg) = 2.21 × weight (kg) × (target Hb − actual Hb) + 1000 (for stores). The expected haemoglobin response is 0.7–1 g/dL per week, with maximal response in 4–9 weeks.

Advantages

• Certainty that the full calculated dose is delivered and the stores are replenished.
• Useful when oral therapy has failed or there is too little time before delivery.
• Total-dose infusion completes treatment in a single sitting and is less costly than repeated IM injections.

Side-effects and Limitations

Local pain and skin staining (IM), flushing, nausea, fever, arthralgia, and rarely anaphylaxis (especially iron dextran). It cannot raise haemoglobin faster than oral iron once absorption is adequate, and the peak response still takes several weeks — so it does not replace transfusion when rapid correction is needed.

A Note on When — and WHY — Iron Must Be Given Parenterally

A useful point is understanding why the oral route, which is cheap and safe, is sometimes abandoned in favour of parenteral iron. Oral iron is first-line, but it has real limitations: only a small fraction is absorbed; absorption is impaired by tea, calcium, phytates and antacids; and the gastrointestinal side-effects (nausea, epigastric pain, constipation, black stools) are so common that many women simply stop taking it — so the rise in haemoglobin is slow (at best about 0.8 g/dL per week) and depends entirely on compliance. Parenteral iron is therefore chosen when oral iron cannot work, or cannot work fast enough: intolerance or non-compliance; malabsorption; severe anaemia; and — the commonest reason — anaemia diagnosed late in pregnancy, when there is insufficient time before delivery for oral iron to correct it. Its advantages are that it replenishes the stores completely and predictably, in a calculated dose, without depending on the gut or on the patient. Modern preparations (iron sucrose, ferric carboxymaltose) have largely replaced iron dextran, whose risk of anaphylaxis was the great drawback. Understanding that parenteral iron buys certainty and speed — not superior efficacy — explains exactly when it is indicated, and why it is not used routinely.

Contraindicated in the first trimester and in active infection.

• Indications for parenteral iron: intolerance or non-compliance with oral iron, malabsorption, severe anaemia, or anaemia diagnosed late in pregnancy.
• Preparations: iron sucrose (200 mg per sitting, repeated) and ferric carboxymaltose (large single dose); iron dextran carries a risk of anaphylaxis.
• Calculate the dose from the haemoglobin deficit plus stores; expect a reticulocytosis by day 5–10 and a rise of ~1 g/dL per fortnight.
• Contraindicated in the first trimester, in infection, and in anaemia not due to iron deficiency.
• Blood transfusion is reserved for severe anaemia near term, decompensation/heart failure, or associated haemorrhage.