Re: Checking urinary Cl- in metabolic alkalosis:
One of the functions of Cl- is to accompany Na+ during its reasborption by the kidney. Cl- is the most abundant anion and makes a good partner when the kidney wants to reabsorb Na+. The 2nd most abundant anion, HCO3-, can also accompany Na+, but is generally not used for this purpose unless Cl- is in short supply.
In a metabolic alkalosis, there is always the possibility the high serum HCO3- is due to excessive reabsorption (along with Na+) by the kidney. This would occur because of a low amount of Cl-. Therefore, checking the urinary Cl- excretion would tip you off to low amounts of Cl-. Low urinary Cl- concentrations in the setting of a high serum bicarbonate would suggest this mechanism. The treatment would be to simply restore the Cl- levels in the body, with either NaCl (salt tabs, saline), or KCl.
Re: Checking urinary K+ levels:
We generally do not check spot urinary K+ levels (unlike other spot urine levels). Spot urinary K+ levels are not a good reflection of the amount of K+ that is being excreted. The reason for this is the location in which K+ is secreted -- the distal tubule. H2O enters the urinary space near the distal tubule, and thus can alter the urine K+ concentration. Since H2O and K+ secretion are not directly related to one another (unlike H2O and Na+ secretion), you could be fooled into thinking that a low urine K+ signifies low excretion of K+.
We have a short 10-Minute Rounds video about this here.