AIM: To study the effects of phototherapy on erythrocyte haemolysis in vitro and to determine possible differences in sensitivity to phototherapy between normal erythrocytes and spherocytes.
METHODS: Erythrocytes from four normal healthy donors and two donors with hereditary spherocytosis were treated with bilirubin (160 microM) in the presence of human serum albumin in the molecular ratio bilirubin/albumin 0.8. Treated cells were maintained either in the dark or in blue light (450 nm, 8 mW/cm2, 30 min). The experimental light dose was comparable to 2 h of clinical phototherapy. The osmotic fragility of the treated cells was measured by scoring haemolysis in hypo-osmolar solutions (0.10-0.90% NaCl). The sensitivity to photohaemolysis of cells pre-treated with bilirubin (BR) and/or phototherapy was tested by exposing the cell suspensions to haematoporphyrin and UVA radiation. The delayed (18 h) photohaemolysis was measured by spectrophotometry.
RESULTS: Osmotic fragility, expressed as percentage haemolysis, of normal erythrocytes was more than doubled in the presence of BR combined with phototherapy (n = 6, p < 0.05). In contrast, osmotic fragility of spherocytes was unaffected by either treatment (n = 8, p < 0.05). Increased photohaemolysis was seen in spherocytes treated with BR (n = 13, p < 0.05), phototherapy (n = 13, p < 0.05) and a combination of the two agents (n = 13, p < 0.05) compared with spherocytes without BR in the dark (n = 6).
CONCLUSION: Bilirubin may make the plasma membrane of normal erythrocytes more fragile. Newborns with hereditary spherocytosis may be sensitive to phototherapy.

Effects of bilirubin and phototherapy on osmotic fragility and haematoporphyrin-induced photohaemolysis of normal erythrocytes and spherocytes.
Roll EB, Christensen T, Gederaas OA.
Acta Paediatr. 2005 Oct;94(10):1443–7.