DNA testing can be pretty accurate, but it depends on how many sequences they test. If you read my test procedure explanation you will understand why DNA is used.
First, Carbohydrates and lipids are pretty much identical between all people. A few people have metabolic disorders that result in a build up of certain carbs, but generally it would be useless to test for the specific carb or lipid present, because they are the same for everyone.
Proteins do differ among people, but not as much as DNA does. DNA provides the information telling ribosomes how to put proteins together in people, but only a small portion of DNA is ever tranlsated to protein. Most of your DNA is junk, and the big DNA differences between two people are in this junk DNA, so that is what is used for DNA testing.
This is an explanation of how it is typically done:
The way DNA testing works is by measuring the lengths of DNA segments between two endonuclease cleavage sites. An endonuclease is an enzyme that will cleave DNA at a particular sequence of bases. Some specific cleavage sites exist in the DNA of all people. In between two specific cleavage sites, you may have a sequence of tandem repeat DNA. For example le us say your DNA looks like this:
(Cleave) gene1-TGTGTGTGTGTG-gene2 (Cleave)
We will denote this length of tandem repeat segment as (TG)6. My DNA may have a tandem repeat segment may be (TG)10, in which case the segment of DNA between the two cleavage points is longer for me than it is for you. This additional length also makes it heavier.
Now let us make a suspend lots of copies of this particular segment of your DNA in a fluid and make another suspension with my DNA.
Let us also say that we know that everyone has a sequence of TG repeats at least 6 repeats long. The complement to this sequence is:
ACACACACACAC -> (AC)6
We could create a radio labelled (AC)6 probe and sprinkle it into the suspension with your DNA, the suspension with my DNA, and a suspension with unknown DNA. This probe would bind to the DNA in all three solutions, but my DNA segment would still be heavier than yours, because it contains 10 repeats, even if only 6 are bound to the probe.
We now place some of the suspension of your DNA at a point on an adsorbent surface. The suspnsion will travel along the surface, but at some point the DNA will stick to the surface. If it is heavier it will stick sooner. With good equipment all DNA of a certain length will stick at the same spot (or short line)
Since my DNA is heavier than yours, mine will travel less far. The unknown DNA will travel either as far as your DNA does or as far as my DNA does, and that will tell whose DNA it is.
The way you view the DNA on the adsorbent surface is with a film that will darken when exposed to our radio labelled probe. You place this film on top of the adsorbent bed with the DNA samples and three small spots (or short lines) will show. The distance the unknown DNA travelled in comparison to your DNA and mine tells us whose DNA it is.
Investigators do this with evidence; only they use many different tandem repeat segments scattered throughout the genome. The more segments used, the lower the probability of a coincidence.