Tests
1st tests to order
CBC
Test
This test can be useful to support a suspicion of blood loss, but the result will vary depending on the clinical situation.
With chronic blood loss or after some time has passed following an acute bleed, the hematocrit will be low.
The hematocrit can be high in pure extracellular fluid volume depletion due to the reduction in plasma volume.
Repeated hemoglobin and/or hematocrit measurements is recommended as a laboratory marker for bleeding, as an initial value in the normal range may mask early-phase bleeding.[29]
Result
normal or decreased hematocrit; normal or low hemoglobin
serum electrolytes
Test
Electrolyte abnormalities are common in volume depletion and help elucidate the cause (e.g., hypokalemia in diarrhea, hyperglycemia in osmotic diuresis, and hypernatremia in dehydration with volume depletion).
Result
hyper- or hypokalemia; hypernatremia
blood glucose
Test
In cases of osmotic diuresis associated with hyperglycemia, blood glucose will be elevated.
Result
normal or elevated
BUN
Test
This finding infers poor renal blood flow, and prerenal azotemia often accompanies volume depletion.
However, BUN is also high in gastrointestinal bleeding and hypercatabolic states, and with glucocorticoid therapy.[11]
Result
elevated
serum creatinine
Test
Can be elevated in many instances but is often high in the setting of severe volume depletion due to decreased blood flow to the kidneys and prerenal azotemia or ischemic acute renal failure.
Result
elevated
lactate
Test
For adults suspected of having sepsis, Surviving Sepsis Campaign guidelines suggest measuring blood lactate.[27] Serum lactate is most reliably assessed using an ABG sample.[28] However, in practice, a venous blood gas sample is usually used, as it is generally easier and quicker to obtain compared with ABG. Most patients do not undergo ABG sampling unless there is respiratory compromise. The presence of an elevated or normal lactate level increases or decreases, respectively, the likelihood of a final diagnosis of sepsis in patients with suspected sepsis. Blood lactate is recommended in hemorrhagic shock as a sensitive test to estimate and monitor tissue hypoperfusion from hypotension and bleeding.[29] The determination of lactate may be particularly important in penetrating trauma, where vital signs, such as blood pressure, heart rate and respiratory rate, do not reliably reflect the severity of injury.[29][30] Serum lactate level should be interpreted considering the clinical context and other causes of elevated lactate.[27]
Result
elevated
procalcitonin
Test
Where available, measurement of serum procalcitonin may be considered in patients with sepsis to guide the duration of antibiotic therapy. For adults with an initial diagnosis of sepsis or septic shock and adequate source control where optimal duration of therapy is unclear, procalcitonin and clinical evaluation are recommended to decide when to discontinue antimicrobials.[27] However, evidence for the prognostic value of procalcitonin alone is unclear, and its use in the identification of sepsis is excluded from many guidelines.[27][36][37][38] Changes in procalcitonin levels may occur later than that of lactate, although changes in both markers combined are highly predictive of outcome between 24 and 48 hours.[39] Other proinflammatory states, such as acute pancreatitis, trauma, major surgery, and burns, can also increase procalcitonin.[40]
Result
elevated
urinalysis
random urine sodium
Test
The urine sodium is low in volume depletion as the kidney conserves sodium and water to maintain extracellular volume.[3]
This test will not always be helpful in individuals with chronic kidney disease, patients receiving diuretics, or during states of osmotic diuresis, metabolic alkalosis, or hypoaldosteronism.
Inappropriately elevated urine sodium in the setting of apparent volume depletion would support renal salt wasting or diuretic use.[3]
This test is not useful in dealing with hemorrhage, as the urgency of treatment precludes the time required to obtain the result.[5][11]
Result
<20 mEq/L
fractional excretion of sodium (FENa)
Test
Measures the percentage of sodium excreted in the urine factored by the amount filtered.[3]
The formula is (urinary sodium x plasma creatinine) divided by (plasma sodium x urinary creatinine). This number is then multiplied by 100 to achieve a percent.[33]
It is helpful in diagnosing an etiology for acute renal failure. A low value (<1%) indicates that most of the sodium delivered to the kidney is reabsorbed as is seen in volume depletion and prerenal azotemia. A value >1% indicates that the kidney is not avidly reabsorbing sodium, either because of diuretic therapy or an intrinsic renal defect, such as acute tubular necrosis. Low FENa may also be seen in glomerulonephritis, hepatorenal syndrome, renal allograft rejection, and contrast-induced acute kidney injury.[33]
Result
<1%
random urine chloride
Test
Result is usually similar to urine sodium, because chloride is reabsorbed with sodium to maintain extracellular volume.
However, in metabolic alkalosis from vomiting there is bicarbonaturia, and urine sodium is lost as the cation along with the bicarbonate anion. Therefore, despite volume depletion, individuals with metabolic alkalosis may have inappropriately high urine sodium. In this setting, low urine chloride (<20 mEq/L) is a much more reliable indicator of hypovolemia.[5][11]
Result
<20 mEq/L
random urine creatinine
Test
Required for calculation of fractional excretion of sodium and urea.
Result
elevated
random urine osmolality
Test
Occurs due to retention of water in the kidney, mediated by antidiuretic hormone, in response to volume depletion.
Values in this range will not be seen if the urinary concentrating ability is impaired by underlying kidney disease, diuretics, osmotic diuresis, hypokalemia, hypercalcemia, or central or nephrogenic diabetes insipidus.
Result
>450 mOsm/kg
rectal exam and fecal occult blood test
Test
Rectal exam may reveal hematochezia (lower gastrointestinal [GI] bleed) or melena (upper GI bleed). Fecal occult blood testing may confirm more chronic GI blood loss.
Result
positive with GI blood loss
Tests to consider
urine urea/fractional excretion of urea (FE urea)
Test
Similar to FENa, it is used to differentiate prerenal causes of acute renal failure from intrinsic or postrenal causes.
The formula is (urinary urea x plasma creatinine) divided by (plasma urea x urinary creatinine). This number is then multiplied by 100 to achieve a percent.
FE urea <35% suggests prerenal azotemia and is helpful if the patient has had diuretic exposure. In this case, the FE urea may be low even if the FENa has been increased by the diuretic drug.
Result
<35%
arterial blood gas (ABG) or venous blood gas (VBG)
Test
May be obtained to help assess the acid-base status of the patient. Hypotension and effective volume depletion from shock often cause lactic acidosis. Lactate levels are most reliably assessed using an ABG sample.[28] However, in practice, a VBG sample is usually used, as it is generally easier and quicker to obtain compared with ABG. Most patients do not undergo ABG sampling unless there is respiratory compromise.
Result
metabolic acidosis or alkalosis
nasogastric lavage
Test
Should be the initial maneuver in suspected gastrointestinal bleeding. The return of blood supports an upper gastrointestinal source of bleed.
Result
positive for blood
stool cultures
Test
Can establish infectious etiology in cases of severe diarrhea.
Result
may show growth of bacteria, toxins, or parasites
abdominal ultrasound
Test
Depending on clinical situation, availability of equipment and expertise of staff may be appropriate initial investigation if intra-abdominal injury or fluid collection is suspected. May reveal presence of fluid in the abdominal cavity but a CT scan is usually required for a more definitive determination of etiology.
Result
intra-abdominal and/or intraluminal fluid may be seen
abdominal CT scan
Test
Depending on clinical scenario, can provide evidence for third-space sequestration or internal bleeding as a cause of volume loss in the appropriate setting.
Result
intra-abdominal and/or intraluminal fluid may be seen
upper gastrointestinal endoscopy
Test
Can be diagnostic and therapeutic for a gastric or duodenal source of gastrointestinal bleeding.
Result
source of bleeding may be identified
colonoscopy
Test
Can be diagnostic and therapeutic for a lower gastrointestinal source of bleeding.
Result
bleeding identified
Emerging tests
saliva osmolality
Test
In older adult patients the diagnosis of dehydration is difficult. Urinary markers of dehydration such as specific gravity, urine color, and urinary osmolality have low diagnostic accuracy.[41] The reference standard for the assessment of hydration in older patients is serum or plasma osmolality; however, these tests are too invasive for day-to-day monitoring of dehydration in an outpatient setting.
Result
>100 mmol/kg
point-of-care ultrasound (POCUS)
Test
Can be used as a quick and noninvasive bedside tool to assess volume in any setting including the emergency department and intensive care unit. The yield of POCUS is dependent on the operator.
Result
Inferior vena cava (IVC) diameter of less than 1.5 cm and compressibility more than 50% indicates volume depletion; IVC diameter more than 2.1 cm and compressibility less than 50% indicates high right atrial pressure and volume overload; B lines in the lung indicate fluid overload
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