Inwood suggests that the use of insulin needles and highconcentration heparin may reduce hematoma formation. The incidence of severe hematoma formation from prophylactic low-dose heparin is sufficiently low that a reduction in frequency with this technique may be difficult to substantiate. A 13-mm (0.5-in) 25-gauge needle is recommended usually for injection of heparin. I would not expect that this minor difference in length or caliber would affect the rate of hematoma formation. I do not know whether injection with the 13-mm 25-gauge needle is less well tolerated by patients than injection with insulin needles.
In most studies of low-dose heparin, a concentration of 25 000 IU/mL has been used. The injected volume is 0.2 mL. In other studies,1 concentrations from 1000 to 10000 IU/mL have been used. Currently, many institutions use 5000-IU/mL single-dose vials for low-dose heparin prophylaxis. Lower volume injections may be less painful to patients, but it