Plaque is like money

In case anyone missed this in the June, 2007 Track Your Plaque Newsletter, I'm again posting how we calculate the annual rate of score increase, should it occur.

For instance, say your score in January, 2005, is 100. In November, 2006, you undergo another scan and the score is 140. Obviously, your score has increased an undesirable 40%. But what is the annual rate of score increase, the amount of increase per year?

In this example, the annual rate of score increase is 19%--not anywhere near as bad as the 40% that can scare the heck out of you.

Obviously, the best rate of heart scan score increase is a negative number, i.e., a drop in score from, say 100, to 60. You might even eliminate the need for this calculation altogether if you drop your score.

Nonetheless, whenever there is a score increase over an uneven period of time, a fraction of year(s), this is the method we use to annualize the calculation. The equation we use is a modified form of the annual compound interest equation using continuous compounding, since that’s how coronary atherosclerotic plaque grows--just like money. The difference is, of course, is that while you might want more money, you certainly don't want more plaque.

You will need a calculator for this calculation, one with an exponential “y to the power x” function. For ease, calculate "1/t first, then use it as the “x” exponent on your yx function and "(score 2 / score 1)" as the "y".

Annual rate of plaque growth (APG) = ( score 2 / score 1 ) 1/t - 1

Multiply the result by 100 to yield a percent.

Score 1” is your 1st heart scan score, “score 2” is your 2nd (or any subsequent heart scan score); “t” is the amount of time between the two scans expressed in years in decimal form. Time between scans should be expressed in years or fractions of years. To obtain the time interval in fractions of years, simply divide the number of months between scans by 12 (e.g., 18 months / 12 = 1.5 years ; 22 months / 12 = 1.83 years).

It’s not as tricky as it looks. For example, if your first heart scan score is 300 and your next scan 16 months later (or 16/12 = 1.33 years) is 372, then:

Annual rate of plaque growth (APG) = ( 372 / 300 ) 1/1.33 - 1 = 0.175

Multiply 0.175 x 100 = 17.5% annual rate of plaque growth

Some scan centers will do the calculation for you as part of a repeat scan. However, the equation can be used if you're left on your own, or if you go to a different scan center. If this is too much effort, perhaps it's just another reason to add to the list of reasons to drop your heart scan score!