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08/23/2011 - uric acid in SIADH and routine labs

posted Sep 28, 2011, 5:20 PM by Chief Resident   [ updated Sep 28, 2011, 5:22 PM by Purnema Madahar ]
Now that things on the floor are hopefully running more smoothly and both you interns and residents have settled into your roles, it might be time to be thoughtful about some of our practices. One of them is the practice of routine labs. There's not a lot of data on the utilitly or harm of routine labs, but you should keep a few things in mind:

1. Are you using the data you gather? Like with any test, ask yourself why you need the lab, what do you expect and how will it influence your decisions?
2. Can you realistically expect a change? Many labs we're ordering daily are very unlikely to be affected by the disease of treament of the particular patient. So why follow them?
3. Harm to patients: interrupted sleep, invasive procedure, blood loss (not insignificant in a sick person with reduced erythropoesis)
4. Error: like all tests, normal values on labs are determined by cutoffs on a curve, inherently leaving room for false positive results; the more labs you draw, the higher the chance that one will be falsely abnormal;
5. Overburdening of the lab and phlebotomist, introducing room for even more errors.
6. Cost

I encourage you to challenge any resident or attending that demands daily, routine labs; particularly low yield labs are Mg, P, LFT, coags. For a more thoughtful piece, read the attached paper entitled 'The unbearable lightness of diagnostic testing' .

In noon conference, Joel Neugarten tought us about hyponatremia. He wanted to make sure that we at least took one thing away from the conference: in SIADH, as opposed to hypo/hypervolemic hyponatremia, serum uric acid is low, usually under 3, due to increased excretion of uric acid. Review articles on the work-up of SIADH put this number at 4, which seems to stem from the attached 1979 NEJM paper, summarized in one of the reviews: 'Beck reported that hyponatremia secondary to SIADH is generally associated with a serum uric acid level <4 mg/dl; however, values are >5 mg/dl in patients with hyponatremia associated with a decrease in the EV (effective volume).'

From a paper (attached) that primarily looked at the utility of the fractional excretion of uric acid (FE-UA) in patients with hyponatremia due to SIADH who are on diuretics, it seems that the sensitivity and specificity of uric acid < 4 for SIADH in patients not on diuretics is 0.83 and 0.83. In this study, 86 consecutive hyponatremic patients were classified based on their history, clinical evaluation, osmolality, and saline response to isotonic saline into a SIADH and a non-SIADH group.