may i make an educated guess about the patient's chloride?

the short answer is: NO
and the long answer is:

chloride is the dominant anion. if you want to know with something close to a clinically meaningful precision, which pathological acids a patient is being subjected to, you need to know at least the big physiological players on the anion side: chloride and bicarbonate.

bicarbonate is easy to calculate from the mass equation joining PCO2, pH and HCO3-, otherwise known as the Henderson-Hasselbalch equation.
(a note of caution: this is the actual bicarbonate, that found in the patient's serum, not the somewhat artificial "standard bicarbonate".)

chloride, though, can only be measured!
any single mmol/l deviation is felt as a pH change - try and take one of the already registered cases, and press on the pHcalc button at the bottom of the page. you can then sequentially change the chloride level and follow the pH changes.
this gets much more pronounced, once the serum already is in the frankly acidotic range! try and look at this example, it is a simulation of 1mEq/l increments in strong anions, chloride being one such anion would have exactly the same effect. it is calculated on the data provided by a typical case of diabetic ketoacidosis at our hospital (case 65 in our database).

consult the glossary for other aspects of acid-base equilibria and the rules and mathematics behind our website:     Glossary