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  1. Guidance, Compliance, & Regulatory Information

Strength Conversion in Drug Listing

Strength Conversion in Drug Listing

A drug strength or concentration of its active ingredients can be expressed in many different ways. In order to standardize the expression of active ingredients in drug listing Structured Product Labeling (SPL) submitted to FDA, the agency has adopted a series of automated validation rules to allow for certain expressions.

For example, the strength of active ingredient is not allowed to be included as a percentage value but can be a concentration of an amount of solute in an amount of solution:

  • As w/w - mass (grams) of solute in mass (100 gm) of solution, like topical creams and ointments
  • As w/v - mass (grams) of solute in a volume (100ml) of solution, like oral liquids
  • As v/v - volume (milliliters) of solute in a volume (100ml) of solution, like alcohol

The strength data element in a listing SPL is designed to accept submissions mostly in concentrations of w/w or w/v format, when the strength of an active ingredient is expressed as a percentage. Percentages must be converted into ratios of w/w or w/v with a value in the numerator and in the denominator including the correct units of measure in order to pass the SPL validation rules.

Exceptions to this rule are alcohol, medical gases and water. Strength of active ingredients in these products can be included in the SPL as v/v and does not need to be converted into w/w or w/v. In cases like alcohol, the value of density can fluctuate depending on the temperature. Converting the strength into w/w or w/v may lead to an incorrect strength. It is therefore recommended that the strength of active ingredient for these products be included as v/v in the listing SPL. 

In most cases, the strength used is that for a single dose following the conventions in the table below. An example of “weight” is milligram, an example of “volume” is milliliter, an example of “time” is hour, and an example of “each” is tablet.

Note that for powders that are reconstituted, the denominator is the volume following reconstitution if the amount is fixed. For example, if 10 mg of a powder in a vial is mixed with 5 ml of diluent, the strength is 10 mg in 5ml. If the reconstituted amount is not fixed, then the denominator is 1. For example, if 10 mg of a powder in a vial is mixed with 5-10 ml of diluent, the strength is 10 mg in 1 vial.

Product

Numerator unit

Denominator unit

Alcohol, Medical Gasses, Water

Volume

Volume

Oral Solid

Weight

Each

Oral Liquid

Weight

Volume

Oral powder for reconstitution with a known volume

Weight

Volume

Oral powder for reconstitution with a variable volume

Weight

Each

Suppository

Weight

Each

Injection Liquid

Weight

Volume

Injection powder for reconstitution with a known volume

Weight

Volume

Injection powder for reconstitution with a variable volume

Weight

Each

Inhaler powder

Weight

Each

Inhaler liquid

Volume

Each

Weight

Volume

Inhaler blister

Weight

Each

Topical cream or ointment

Weight

Weight

Topical gel

Weight

Volume

Weight

Weight

Topical lotion

Weight

Volume

Transdermal patch

Weight

Time

Topical patch

Weight

Weight

Weight

Each

Bulk liquid

Weight

Volume

Bulk solid

Weight

Weight

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Mathematical errors have been detected in many cases with strength conversions in the listing SPLs. One example is the incorrect placement of decimal points when the strength expression is converted from percentage (%) into w/v, w/w or v/v. In some submissions, the units of measurement (for example, grams and milligrams) are used incorrectly, are interchanged, are missing or the calculation itself is done incorrectly. Another error associated with strength conversion can be a data entry error which does not correlate with the actual strength included on the product’s label.

In many of these submissions, no automated validation errors are generated, as there are no reference points which can be made to the labeling information. These errors are later detected through our surveillance projects. If not corrected by the firms, an error in the listing data submitted to the FDA can result in the removal of the incorrect data by FDA from public listing sites as well as possible further actions, such as untitled letters or warning letters.

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