Calculation question, lbs to kg conversion.

[firework]

When you convert lbs to kg, depending on if you use the divided by 2.2 approach or the multiply by 0.454 approach, you can get different answers even after rounding up. For example, a question from the Rxprep question bank:

A physician writes an order for voriconazole 6 mg/kg IVPB Q12H for 24 hours for a patient weighing 150 lbs. How many total milligrams (mg) of voriconazole will the patient receive? Round your answer to the nearest whole number. Do not enter units in your answer; enter only the number.

Correct Answer:
818

If you convert 150 lbs to kg using 150*0.454 (which is actually more accurate than 150/2.2), the answer will be 817.

In the real exam, which is supposed to be right? Will the questions be designed not sensitive to such difference?

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[mike36]

You understand that dividing by 2.2. is the exact same thing as multiplying by 0.454, right? 0.454 is simply 1/2.2.

So 150/2.2 = (150) x (1/2.2) = (150) x (0.454).

Besides, there is no clinical difference between 818 mg and 817 mg. The answer choices would have to be farther apart from each other than just 1 mg.

 

[firework]

1/2.2 equals 0.454545....., it is not exactly 0.454

1 lbs is 0.453592 kg, which is closer to 0.454 than to 1/2.2

I know this is not clinically relevant, but in an exam question such as the one in my original post, you are asked to type the answer yourself, there are no choices. In this scenario, what is the right answer? 818 or 819? Or, will I be punished for choosing one way and not the other?

You understand that dividing by 2.2. is the exact same thing as multiplying by 0.454, right? 0.454 is simply 1/2.2.

So 150/2.2 = (150) x (1/2.2) = (150) x (0.454).

Besides, there is no clinical difference between 818 mg and 817 mg. The answer choices would have to be farther apart from each other than just 1 mg.

 

[mike36]

Are you serious, dude? Actually, 1 kg = 2.2046244 lbs. 1/2.2046244 = 0.453592. It is the same number. We round to get the 2.2, just like we round to get the 0.454. Most conversions that we use are rounded for the sake of convenience. 1 year is not exactly 365 days, and 1 month is not exactly 30 days. We round to get those numbers.

 

[firework]

Can you please do the question in my original post, use 2.2 and then use 0.454, and tell me your answers. In the real test, if your answer is 818 and my answer is 817, who will get the point, will the test be smart enough to give both of us the point, or is there only ONE correct answer? Remember, a lot of questions in the test today ask you to type in your answer, you are not always given choices.

As a matter of fact, I am not just worried about the lbs to kg conversion because if I have to, I can remember not to use 0.454 and use 2.2 instead (which would be ridiculous since 0.454 is actually more accurate but I won't argue, I just want to pass the exam). I am worried there are other hidden places that I am not aware of.

Are you serious, dude? Actually, 1 kg = 2.2046244 lbs. 1/2.2046244 = 0.453592. It is the same number. We round to get the 2.2, just like we round to get the 0.454. Most conversions that we use are rounded for the sake of convenience. 1 year is not exactly 365 days, and 1 month is not exactly 30 days. We round to get those numbers.

 

[UFPharm2015]

Firework - There is no absolute way anyone can answer your question. I can only go based off what I've used to study for the NAPLEX.

Lb/2.2 = KG seems to work for every practice question i've encountered so far (including free response).

 

[mike36]

Can you please do the question in my original post, use 2.2 and then use 0.454, and tell me your answers. In the real test, if your answer is 818 and my answer is 817, who will get the point, will the test be smart enough to give both of us the point, or is there only ONE correct answer? Remember, a lot of questions in the test today ask you to type in your answer, you are not always given choices.

As a matter of fact, I am not just worried about the lbs to kg conversion because if I have to, I can remember not to use 0.454 and use 2.2 instead (which would be ridiculous since 0.454 is actually more accurate but I won't argue, I just want to pass the exam). I am worried there are other hidden places that I am not aware of.

They are both rounded numbers, so it doesn't matter which way you do it. Yes, I understand that you get a difference of 1 mg when you do the problem above. As I have already said, it is not clinically relevant. Do you really think that NAPLEX would give credit for 817 mg and not for 818 mg, or vice versa? It's the same number for all practical purposes. I obviously cannot speak for NAPLEX, no one on here can, but it makes sense that they would not be that nitpicky. This is a non-issue; you should really focus on more important things.

 

[firework]

Firework - There is no absolute way anyone can answer your question. I can only go based off what I've used to study for the NAPLEX.

Lb/2.2 = KG seems to work for every practice question i've encountered so far (including free response).

Thanks, I will go with 2.2 then just for the peace of mind.