Syndrome of Inappropriate ADH Secretion: Sensitivity and Specificity

Introduction: SIADH a major cause of hyponatremia and represents the condition when a euvolemic patient has increased ADH causing inappropriate retention of free water. The diagnosis is important as the treatment is fluid restriction rather than normal saline infusion.

For a review of the diagnosis, see http://cjasn.asnjournals.org/cgi/content/full/3/4/1175 (PMID 18434618)

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Differential Diagnoses include: Hypovolemia

Prevalence

Population / Calculator	Prevalence	Comments / Study / Link
Elderly patients with hyponatremia	31%	110 consecutive cases: 31% SIADH patients, 23% patients with hyponatremia due to diuretics, 18% potomania patients, 15% salt depleted patients, 5% salt depleted SIADH patients, 5% patients with a salt loosing syndrome and 3% patients with hyponatremia of unknown origin Int Urol Nephrol. 2001;32(3):475-93. PMID: 11583374
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The sensitivity and specificity of findings for Syndrome of Inappropriate ADH Secretion are listed below. See the left navigation bar to change the display.

Pre-Test Probability (Prevalence): %.	Post-Test Probability (Predictive Value): %.	Switch to display mode.
Note that calculating probabilities from more than one** finding is inherently inaccurate because findings are not independent. (For example, using two positive findings that share a common pathogenesis is likely to overestimate the true probability.)

Specific Findings
Finding	Result	Sensitivity	Specificity
[ + ] Fractional Excretion of Sodium	+ 0 -	68%	100%
FENa > 0.5% Differential Diagnosis: Hypovolemia Study: Clin J Am Soc Nephrol. 2008 Jul;3(4):1175-84. PMID: 18434618

Sensitive Findings
Finding	Result	Sensitivity	Specificity
[ + ] Urine Sodium	+ 0 -	90%	70%
UNa > 30 indicates SIADH. However, UNa can be high in hypovolemic patients, especially those with low UOP. Differential Diagnosis: Hypovolemia Study: Clin J Am Soc Nephrol. 2008 Jul;3(4):1175-84. PMID: 18434618

Poorly Diagnostic Findings
Finding	Result	Sensitivity	Specificity
[ + ] Plasma Urea	+ 0 -	80%	80%
plasma Urea < 30 mg/dl suggests SIADH (because of dilution) Differential Diagnosis: Hypovolemia Study: Clin J Am Soc Nephrol. 2008 Jul;3(4):1175-84. PMID: 18434618
[ + ] Fractional Excretion of Urea	+ 0 -	52%	80%
FEUrea > 50% Differential Diagnosis: Hypovolemia Study: Clin J Am Soc Nephrol. 2008 Jul;3(4):1175-84. PMID: 18434618
[ + ] Uric Acid	+ 0 -	73%	59%
uric acid < 4mg/dl suggests SIADH (because of dilution) Differential Diagnosis: Hypovolemia Study: Int Urol Nephrol. 2001;32(3):475-93. PMID: 11583374
[ + ] Clinical Assessment of Volume Status	+ 0 -	48%	53%
Study: Am J Med. 1987 Nov;83(5):905-8. PMID: 3674097