Intoxications with increased osmolar gap but normal acid base parameters

Explanation

What is osmolality? It is the concentration of osmotically active substances present in a liquid.

In the case of serum it can be reported in 2 ways. Measured or Calculated.

Measured osmolality is measured by the freezing point lowering method and gives the true osmolality of the solution.

Calculated osmolality is = Na x 2 + (BUN/2.8) + (Glucose/18) and this holds good in most scenarios except the following:

When there is a significant amount of osmotically active substance in blood that is other than Na/BUN/Glucose, the levels of Na/BUN/Glucose adjust to bring the osmolality towards normal but in these cases the calculated osmolality will be much less than the measured osmolality. Such a situation is called as one where there is an osmolar gap. Such osmotically active substances are:

Isopropyl alcohol: No acidosis but severe osmolar gap. Assay is difficult locally in most hospitals. Send out lab.

Methanol (methyl alcohol- gets metabolized to formic acid): Found in local made liquor and rubbing alcohol. Severe acidosis with increased anion gap. Can cause blindness. Actually the first line of treatment is IV Bicarbonate therapy because it prevents blindness due to methanol but in exams I have seen IV ETOH (alcohol) as the required answer because the bicarbonate is not a choice offered.

Paraldehyde: Typically presented as anion gap acidosis but normal osmolar gap.

Ethylene glycol: Gap acidosis + osmolar gap - but what distinguishes it from others is the presence of oxalate crystals in urinalysis.