TOPIC DISCUSSION: SIADH diagnosis

We always struggle to diagnosis this entity in few cases of hyponatremia. Measurement of AVP can prove challenging due to problems of ordering and time it takes to come back. Measurement of copeptin( a 39 amino acid glycopeptide) which is derived from the same precursor peptide as AVP and released in equimolor amounts as AVP might be measurable.  Is that been studied? You bet!

In this one prospective observational study, 106 consecutive hyponatremic patients were classified based on their history, clinical evaluation, and laboratory tests. In patients and 32 healthy control subjects, plasma copeptin concentration and standard biochemical parameters were tested for their utility of diagnosing SIADH.

Plasma copeptin levels were significantly higher in patients with hypo- and hypervolemic hyponatremia compared with SIAD (P < 0.005, respectively) and primary polydipsia (P < 0.001). The copeptin to U-Na ratio differentiated accurately between volume-depleted and normovolemic disorders (area under the receiver-operating characteristic curve 0.88, 95% confidence interval 0.81-0.95; P < 0.001), resulting in a sensitivity and specificity of 85 and 87% if a cutoff value of 30 pmol/mmol was used.  This ratio differentiated between volume-depleted and normovolemic disorders resulting in good specificity and sensitivity in this one study. It did better than copeptin alone as well. Another small swiss study felt that it added very little to the sodium balance information. I guess not much out in the literature. Lets see where this story unfolds and how this becomes real in clinical use.

Ref

http://www.ncbi.nlm.nih.gov/pubmed/18984663

http://www.ncbi.nlm.nih.gov/pubmed/21990032