Interpreting data

Question

[V]

Hi

I am interpreting volcano plot data and when I presented my data to a colleague she explained why a log2(FC) that's negative would mean that the disease group had an increase in abundance of a particular lipid or metabolite but I am having trouble grasping a - = an + change. Now the metabolites with -'s were expected to be higher than the controls so it makes sense from the stand point of having a particular compound have a greater abundance vs. the control group but it just doesn't make sense because negative's typically mean decreases LOL. Examples of compounds I found that make sense when the negative is applied.

                FC     log2(FC)  Adjusted P-value
Carteolol       0.651  -0.620    0.0272
DL-Lactic Acid  0.448  -1.158    1.70E-03

Where it doesn't

Ticlopidine 139.820 7.127   6.79E-03

(the controls had higher antiplatelet and the HCM disease were found to be on therapeutics like carteolol and high DL-lactic acid can indicate congestive heart failure due to the heart over working and building up lactic acid)

So yea anyway can anyone explain this math to me to make sense because when I've looked up easy English examples to explain how a negative log = disease has an increased abundance I got nothing. I'm honestly embarrassed to ask her again and she's away on vacation beyond the due date of this paper sadly. but instead of typing results and just using the information she provided I'd LOVE to actually understand it!

m_D = mean of diseased
m_C = mean of control

FC = m_C/m_D = 0.04    so  m_C = 0.04 x m_D
log2(FC) = log2(0.04) = -4.634    so  log2(m_C) = -4.634 + log2(m_D)

FC > 1 means log2(FC) > 0 and m_C > m_D
FC < 1 means log2(FC) < 0 and m_D > m_C
FC = 1 means log2(FC) = 0 and m_D = m_C

Answer 1

[Ram]

The moment you said negative = more in control, it had to mean that m_D goes in the denominator (which is weird since it should be the numerator). Your logic is internally consistent though so I don't understand which part confuses you.

Instead of numbers like 0.04, try m_C=2, m_D1=1 and m_D2=4.

log2(FC_D1) = log2(2/1) = log2(2) = +1, where D1 has less metabolite than C

log2(FC_D2) = log2(2/4) = log2(1/2) = -1, where D1 has more metabolite than C

[V]

Oh. :) Yea it was the numbers. The decimals must be throwing me off because most have decimals. Thank you!!!!!!!!! :)

[Ram]

I've moved my comment to an answer. Please accept it if it resolved your question.