To convert one to the other accurately, multiply or divide by 18
mg/dL is the bigger number, so divide it by 18 to get mmol/L
mmol/L is the smaller number, so multiply it by 18 to get mg/dL
-
An example:
-
134 mg/dL divided by 18 = 7.4444... which rounds down to 7.4 mmol/L
-
64 mg/dL divided by 18 = 3.5555... which rounds up to 3.6 mmol/L
-
and the other way:
-
11.9 mmol/L multiplied by 18 = 214.2 which rounds down to 214 mg/dL
-
5.6 mmol/L multiplied by 18 = 100.8 which rounds up to 101 mg/dL
mg/dL is normally given to the nearest whole number eg 62
mmol/L is normally given to one decimal place eg 4.3
Notice how the mg/dL figure is always MUCH bigger than the mmol/L one, so it's almost impossible to confuse which units someone is using.
Isn't there an easier way to convert?
With a calculator the accurate conversion is easy, but I don't always have my calculator to hand, and I certainly can't divide by 18 in my head, nor can I remember how to do long division! However...
Using 20 rather than 18 is just fine. This is easy to do in your head:
-
To go one way, just multiply by 10 and double it, or double it and multiply by 10;
-
the other way: remove the last digit and half it.
So:
12.1 mmol/L -> 121 -> 242 mg/dL
and 3.4 mmol/L -> 34 -> 68 mg/dL
and the other way:
280 mg/dL -> 28 -> 14 mmol/L
and 90 -> 9 -> 4.5 mmol/L
Or you can use this table:
Just the find the number nearest the one you want to convert:
| mg/dL |
20 |
30 |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
120 |
130 |
140 |
150 |
160 |
170 |
| mmol/L |
1.1 |
1.7 |
2.2 |
2.8 |
3.3 |
3.9 |
4.4 |
5.0 |
5.6 |
6.1 |
6.6 |
7.2 |
7.8 |
8.3 |
8.9 |
9.4 |
| mg/dL |
170 |
180 |
190 |
200 |
210 |
220 |
230 |
240 |
250 |
300 |
350 |
400 |
450 |
500 |
| mmol/L |
9.4 |
10.0 |
10.6 |
11.1 |
11.5 |
12.2 |
12.8 |
13.3 |
13.9 |
16.6 |
19.4 |
22.2 |
25.0 |
27.8 |
What are the units called?
In some countries (the US and Germany, for example) mg/dL is used (pronounced milligrams per deci-litre, which is thousandth's of a gram per tenth of a litre)
Other countries (Canada, Britain and Scandinavia, for example) use mmol/L (pronounced millimoles per litre). A millimole is a thousandth of a mole, and is 602,253,000,000,000,000,000 molecules of glucose.
There is a further problem with blood glucose readings: most meters give the result as how much glucose is dissolved in a given amount of whole blood. Whole blood is the red stuff that comes out of your finger.
But many laboratories, and some meters, quote the result as how much glucose is in the blood plasma . Plasma is the name for the liquid part of your blood. Blood is not just a red liquid. The "liquid" in your blood is actually clear, like water. Floating around in the liquid are various things, including tiny red blood cells which give the blood its red color. The clear liquid is called plasma, and has many nutrients, including glucose, dissolved in it. (Sometimes a serum value is used: serum is similar to plasma but has the blood clotting factors removed.)
When a doctor takes a blood sample from you, and sends it to a laboratory, the lab may first extract just the plasma from the blood, and just test this for it's glucose content. They may do this by spinning it very fast in a centrifuge, so the blood cells go to one end, and the clear liquid to the other.
The problem is that the clear plasma contains a different amount of glucose from the red whole blood. The plasma reading is very slightly higher than the whole blood reading. So when your doctor quotes a figure, you must check whether it's blood plasma, or whole blood being referred to.
The whole blood figure is 12% lower than the plasma figure.
-
Whole blood is the lower figure, so multiply it by 1.12 to get the equivalent plasma concentration;
-
Plasma is the higher figure, so divide it by 1.12 to get the whole blood figure.
This table gives a rough conversion:
mg/dL
(US)
|
| Whole blood |
20 |
30 |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
120 |
130 |
140 |
150 |
160 |
170 |
| Plasma |
22 |
34 |
45 |
56 |
67 |
78 |
90 |
101 |
112 |
123 |
134 |
146 |
157 |
168 |
179 |
190 |
| Whole blood |
170 |
180 |
190 |
200 |
210 |
220 |
230 |
240 |
250 |
300 |
350 |
400 |
450 |
500 |
| Plasma |
190 |
202 |
213 |
224 |
235 |
246 |
258 |
269 |
280 |
336 |
392 |
448 |
504 |
560 |
|
|
mmol/L
|
| Whole blood |
1.0 |
1.5 |
2.0 |
2.5 |
3.0 |
3.5 |
4.0 |
4.5 |
5.0 |
5.5 |
6.0 |
6.5 |
7.0 |
7.5 |
8.0 |
8.5 |
| Plasma |
1.1 |
1.6 |
2.2 |
2.7 |
3.3 |
3.8 |
4.4 |
4.9 |
5.5 |
6.0 |
6.6 |
7.1 |
7.7 |
8.2 |
8.8 |
9.3 |
| Whole blood |
8.5 |
9.0 |
9.5 |
10.0 |
10.5 |
11.0 |
11.5 |
12.0 |
14.0 |
16.0 |
18.0 |
20.0 |
25.0 |
30.0 |
| Plasma |
9.5 |
10.1 |
10.6 |
11.2 |
11.8 |
12.3 |
12.9 |
13.4 |
15.7 |
17.9 |
20.2 |
22.4 |
28.0 |
33.6 |
|
By John Neale
Reproduced with permission.
