Abstract 52

Children with high white blood cell (WBC) counts in ANLL are at especially high risk for strokes. Both, procoagulant substances from malignant cells as well as leukostasis are accounted as causes for this life threatening complication.

5 patients were treated with exchange transfusion because of WBC counts greater than 250000/µl. Leukemias were diagnosed as ALL (n=1) and M4(n=3) and M5 (n=1) ANLL according to the FAB Classification. Mean cellular volumes of WBC were 82 fl (ALL), 240 - 420 fl (M4 ANLL) and 730 fl (M5 ANLL).

Viscosities of blood suspensions with increasing WBC counts (0 - 280000/µl) were studied by capillary viscosimetry in glass tubes with diameters of 280, 100, 50 and 25 µm. Viscosities of WBC free suspensions (adjusted hematocrit = 0.25 L/L) did not differ among the 5 patients and decreased continuously (mean decrease of viscosity = -46%) with decreasing tube diameter (FLE = Fahræus - Lindquist effect). Increasing WBC count resulted in significantly increased viscosities in all tubes (M5> M4 > ALL). FLE remained to be preserved in ALL and in 2 patients with M4 ANLL. In 25 µm tubes FLE was diminished in one case of M4 ANLL and inversed in M5 ANLL at WBC counts > 120000/µl. Both cases showed the largest WBC volumes among the five patients.

Conclusions: Relevant hyperviscosity occurs in hyperleukosis. Leukostasis in narrow tubes as well as in small vessels may occur especially in ANLL with extremely large WBC.