Formulas

Question 1

Serum Osmolality

Answer 1

2Na + Glu/18 + BUN/2.8 + Etoh/4.6

Lab - Calculated > 10 suggests unmeasured substance

Remember, ISOPROPYL ALCOHOL CAUSES AN OSMOLAR GAP BUT NO ANION GAP

Question 2

Likelihood Ratios

Answer 2

Positive LR = Sensitivity/(100 - Sensitivity)

Negative LR = (1 - Sensitivity)/Specificity

Question 3

Lights’a Criteria

Answer 3

Pleural/Serum Protein > 0.5

Pleural/Serum LDH >0.6

Pleural LDH > 2/3 ULN of serum LDH

If any positive, have an EXUDATE (infection, malignancy, rheumatologic dz, pulmonary embolism, pancreatitis, post-CABG)

Otherwise TRANSUDATE (heart failure, cirrhosis, nephrotic, constrictive pericarditis)