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Light-absorbing properties and osmolarity of indocyanine-green depending on concentration and solvent medium.

PURPOSE
To evaluate the absorption spectrum and osmolarity of three currently used indocyanine green (ICG) products at different concentrations and with different solvent media.


METHODS
The absorption spectrum and osmolarity of three different ICG products (Pulsion, Munich, Germany; Akorn, Buffalo Grove, IL; and Laboratoires SERB, Paris, France) were analyzed. Each ICG was further diluted with balanced salt solution or glucose 5%. Four different concentrations were evaluated: 0.005%, 0.0025%, 0.001%, and 0.00025%. ICG (Pulsion) diluted in viscoelastic material (Healon; Pharmacia, Stockholm, Sweden) was analyzed at a concentration of 0.0025%. The following parameters were measured: absorption spectrum between 400 and 1000 nm, osmolarity, and the emission spectrum of the light source of a commonly used vitrectomy machine (Megatron; Geuder, Heidelberg, Germany).


RESULTS
Independent from the manufacturer, concentrations of 0.005%, 0.0025%, and 0.001% ICG diluted in balanced salt solutions (BSS and BSS Plus; Alcon Pharmaceuticals, Fort Worth, TX) and glucose 5% showed two maxima, one at approximately 700 nm and a second one at 780 nm. There was an increase from zero to maximum absorption between 600 and 700 nm and a return to zero between 800 and 900 nm. The absorption band of ICG diluted in the viscoelastic material was similar to the saline solution (BSS or BSS Plus)-diluted ICG. At lower concentrations of 0.001% or 0.00025%, the peak at 700 nm decreased, forming a shoulder in the curve, whereas the peak at 780 nm remained stable. Osmolarity was in the range of 302 to 313 mOsM for BSS Plus-diluted ICG. When glucose 5% was used for ICG dilution, absorption between 600 and 700 nm decreased, and osmolarity was lower (between 292 and 298 mOsM). The light source emission was between 380 and 760 nm.


CONCLUSIONS
Dilution of ICG using the balanced salt solutions BSS or BSS Plus resulted in a steep increase of absorption starting at 600 nm. In clinical practice, there is an overlap between the absorption band of ICG and the emission curve of the light source, resulting in a possible photosensitizing effect, especially at higher ICG concentrations. This effect becomes less likely with decreasing ICG concentrations or when glucose 5% is used as a solvent medium.