One of the common issues facing patients and podiatrists alike is when it comes to creams and ointments - how much should you apply to the skin? When we issue a patient with a tube of terbinafine for example, we may say “use it once a day” but how often do we advise how much to use? Words like “sparingly”, “generously” or “liberally” are hard to define in terms of quantity and ultimately it may result in the patient using too little or too much. This may be of particular concern when using drugs like potent topical steroids or just optimizing the use of emollients or other medicaments.
Figure 1: The fingertip unit – roughly equates to 0.5 gram of a medicament
In 1989, Prof. Andrew Findlay, Prof. Keith Harding and Peter Edwards published the first mention of the “Fingertip Unit” (or FTU for short) in the Lancet (1) with a full paper following in 1991 (2). It worked based on the average length of the distal phalanx of the little finger in an adult (which is on average 2.5 cm in men and 2.3 cm in women). When using any ointment, the natural thing to do is to squeeze the cream out of the tube held in one hand onto the fingertip of the opposite hand. In this case the user is required to squeeze out enough to run from the crease of the base of the distal phalanx to the tip (figure 1) of the little finger described as the “fingertip unit”.
Based on this principle, when an FTU of a cream is extruded from a 5 mm nozzle (a fairly standard diameter in the industry), around 0.5 g (0.49 g in males and 0.43 g in females to be exact) of the medicament is expressed onto the finger. From this quantity, a more accurate dosing can be calculated by the user to ensure that they are using an appropriate dose.
In podiatry, I have applied this principle. Emollients are a common prescription for our patients and the FTU is useful. A dry foot requires around 2 g of emollient a day, very dry feet as much as 4 g. The FTU allows a much easier way to communicate this. In this example 2 grams is 4 fingertip units of emollient. In addition, when prescribing say, a 500 g pump of emollient cream, it allows a rough calculation of how long it might be expected to last the patient.
Another example is anti-fungal cream. 1 FTU equates to about 0.5 g which is enough to apply to one foot covering every area of skin below the ankle line. So, for most patients a 15g tube of terbinafine (applied once daily) should last a fortnight for a pair of fungal feet based on 1g a day being applied. Of course, clotrimazole and miconazole are twice daily applications and consequently 2g a day would be used meaning they would only last a week on this basis. So, despite their cheaper cost per gram than terbinafine, they may work out more costly in the longer term as more quantity is needed.
References
1. Finlay AY, Edwards PH, Harding KG. "Fingertip unit" in dermatology. Lancet. 1989;2(8655):155.
2. Long CC, Finlay AY. The finger-tip unit--a new practical measure. Clin Exp Dermatol. 1991;16(6):444-7.