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Clinical Overview Emergency Medicine Internal Medicine/Endocrinology

Urology/Renal Function

Renal Malfunction Differential Diagnosis:Urine and serum osmolality respond differently to water overload, water deprivation, diuretics, antidiuretic hormone (ADH) and insulin.

Uremia: The difference between measured and calculated osmolalities can determine the accumulation of unmeasured metabolites in the blood. Once baseline electrolyte levels are established, osmolality can be used to monitor the progress of the uremic patient.

Osmolal & Free Water Clearance: Clearance can be calculated immediately from urine and serum osmolality, providing the earliest marker of tubular damage.

Renal Dialysis: In patients with known renal disease, elevated serum osmolality can indicate the need for dialysis. During therapy, dialystate osmolality can be used to monitor progress.

Renal Function: An osmometer is an indispensable aid to understanding the differing responses of serum and urine to water load and deprivation, and to making an appropriate diagnosis. Monitoring the progress of therapy with diuretics, ADH and insulin.

In patients with normal blood volume, a simultaneous determination of serum and urine osmolality will help establish an SIADH diagnosis. In hypovolemic treatment with ADH, an osmometer provides a rapid measure of patient response upon which an adjustment of therapy can be gauged. Free water clearance can be calculated immediately, providing the earliest indication and prevention of tubular damage.

An osmometer can help differentiate psychogenic polyuria from diabetes insipidus, and to measure the effects of humoral factors in the etiology of the disease.

In patients with known renal disease, elevated serum osmolality can indicate the need for dialysis. Dialysate monitoring during therapy can help to minimize the duration of treatment.

The provision of a calculated osmolality, by an automated electrolyte analyzer, does not absolve the pathologist of the need to measure osmolality directly. To the contrary, much useful information can be gained by an investigation of the osmolal gap. The accumulation of unmeasured metabolites in the blood can be an early indication of uremia, or of potentially toxic levels of theraputic drugs. Salycalytes are a prime candidate.

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