Introduction
The preparation of eye drops has decreased significantly in the pharmacy because of the requirements for sterility by the Pharmacopoeia. Nevertheless, the TMF still has a composition for artificial tears, together with a preparation method, which can be applied in pharmacy environment. This procedure can be applied to other requirements such as e.g. for an eye drop without preservative, which is sometimes offered in the pharmacy.
Often the pharmacist encounters problems in calculating the amount of NaCl or glucose, which should be added to make the Eyedrop isotonic.
The most appropriate method for this is to use the NaCl – equivalent or E value, which BV can be found in the Merck Index.
By multiplying the prescribed quantity with the E value, the active ingredient is converted to an osmotically equivalent quantity of NaCl (= A mg). Knowing that for 10 ml of eye drop 90 mg NaCl is required for an ISO Osmotosche solution, the amount of NaCl still to be added can be calculated by subtracting the quantity A from 90.
When the E value is not available, a second convenient method can be used using the value of the isotonic concentration of the prescribed medicine together with the isotonic concentration of the excipient used, which is usually NaCl (0.9%) Glucose (5%) .
Example with E value
R/ Atropine sulfate 1% F.S.A. 10 ml isotonic eyedrop, without preservative
E value = 0.14
So
A/ 100 mg X 0.14 = 14 mg
B/ 90 mg – 14 mg = 76 mg NaCl to be added.
Example with isotonic concentration
The isotonic concentration of atropine sulfate = 8.85% and of NaCl 0.9%
So
A/ 8850 mg give 100 ml of isotonic solution
, 100 mg 1.13 ml isotonic solution
B/ 10 – 1.13 = 8.87 ml must still be made Isotone
Here is 8.87 x (900 mg/100) = 79.83 mg NaCl required.
Note: Both methods give a slightly different negligible result. This difference has to do with whether or not to consider the ionization degree.