This study measured the insulin concentration (Ins_[C]) of NPH insulin in vials and cartridges from different companies after either resuspension (R+) or not (R−; in the clear/cloudy phases of unsuspended NPH).

Measurements included Ins_[C] in NPH(R+) and in the clear/cloudy phases of NPH(R−), and the time needed to resuspend NPH and time for NPH(R+) to separate again into clear/cloudy parts.

In vials of NPH(R+) (assumed to be 100%), Ins_[C] in the clear phase of NPH(R−) was<1%, but 230±41% and 234±54% in the cloudy phases of Novo Nordisk and Eli Lilly NPH, respectively. Likewise, in pen cartridges, Ins_[C] in the clear phase of NPH(R−) was<1%, but 182±33%, 204±22% and 229±62% in the cloudy phases of Novo, Lilly and Sanofi NPH. Time needed to resuspend NPH (spent in tipping) in vials was brief with both Novo (5±1s) and Lilly NPH (6±1s), but longer with all pen cartridges (50±8s, 40±6s and 30±4s from Novo, Lilly and Sanofi, respectively; P=0.022). Time required for 50% separation into cloudy and clear parts of NPH was longer with Novo (60±7min) vs. Lilly (18±3min) in vials (P=0.021), and affected by temperature, but not by the different diameter sizes of the vials. With pen cartridges, separation into clear and cloudy parts was significantly faster than in vials (P<0.01).

Ins_[C] in NPH preparations varies depending on their resuspension or not. Thus, subcutaneous injection of the same number of units of NPH in patients with diabetes may deliver different amounts of insulin depending on its prior NPH resuspension.