Antibiotic Significance

Our puppy recently had a dose of the squits that had to be treated with an antibiotic (ethrythromycin, if you’re interested). This products is available as a bottle of granules that has to be made up with a small volume of water. Now, here’s where it gets interesting from a significant figures perspective.

The plastic bottle in which the granules came is approximately 140 ml, but it is the request to add 109 ml of water to formulate the medication, that is most bizarre. 109? Not 110? Will that single milliliter of water make a whole lot of difference to the formulation? Absolutely not! If it were made up to 110 +/- 10 ml, that would be perfectly okay. So, why do they insist on the pharmacist adding precisely 109 ml? Who knows? It’s one of the eternal mysteries of significant figures. I doubt that even pharmacists actually have a graduated measuring cylinder that they read to the nearest ml, in the first place.

But, that’s only the beginning. When reconstituted, the bottle is made up to 140ml. That’s 2 daily doses of 5 ml (a teaspoon) for two weeks, right? Wrong! The printed guidelines say to finish up the whole bottle, yet our veterinarian gave us the instruction to dose our puppy with 5 ml in the morning and 5 ml in the evening, for TEN days. So, what should we have done with the excess? Again, who knows?

There’s more. Those guidelines suggest storing the erythromycin suspension in the refrigerator. That’s easy enough. But, they also say that you must use the suspension within 7 days. How’s that? Our vet tells us to dose the puppy for ten days, there’s enough in the bottle for fourteen days and we are supposed to use it all up, yet the medication expires in a week of first formulating it. Thankfully, we didn’t get too hung up on all these figures and followed the 10-day rule for antibiotics.

The puppy is fine now, thanks for asking.