Investigations
1st investigations to order
FBC
Test
This test can be useful to support a suspicion of blood loss, but the result will vary depending on the clinical situation.
In acute blood loss, both red blood cells and plasma are lost in parallel, and the loss of whole blood may not be reflected as a drop in haematocrit.
With chronic blood loss or after some time has passed following an acute bleed, the haematocrit will be low.
The haematocrit can be high in pure extracellular fluid volume depletion due to the reduction in plasma volume.
Result
normal or decreased haematocrit; normal or low haemoglobin
serum electrolytes
Test
Electrolyte abnormalities are common in volume depletion and help to elucidate the cause (e.g., hypokalaemia in diarrhoea, hyperglycaemia in osmotic diuresis, and hypernatraemia in dehydration with volume depletion).
Result
hyper- or hypokalaemia; hypernatraemia
blood glucose
Test
In cases of osmotic diuresis associated with hyperglycaemia, blood glucose will be elevated.
Result
normal or elevated
serum urea
Test
This finding infers poor renal blood flow, and prerenal azotaemia often accompanies volume depletion.
However, urea is also high in gastrointestinal bleeding and hypercatabolic states, and with glucocorticoid therapy.[9]
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 azotaemia or ischaemic acute renal failure.
Result
elevated
lactate
Test
For adults suspected of having sepsis, Surviving Sepsis Campaign guidelines suggest measuring blood lactate.[14] The presence of an elevated or normal lactate level significantly increases or decreases, respectively, the likelihood of a final diagnosis of sepsis in patients with suspected sepsis. However, lactate is neither sensitive nor specific enough to rule in or rule out the diagnosis on its own. Serum lactate level should be interpreted considering the clinical context and other causes of elevated lactate.[14]
Result
elevated
procalcitonin
Test
Where available, measurement of serum procalcitonin should be considered in all 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.[14] However, evidence for the prognostic value of procalcitonin alone is unclear, and its use in the identification of sepsis is excluded from many guidelines.[14][18][19][20] 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.[21] Other pro-inflammatory states, such as acute pancreatitis, trauma, major surgery, and burns, can also increase procalcitonin.[22]
Result
elevated
urinalysis
Test
High specific gravity suggests dehydration.
Result
may show high specific gravity
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 underlying 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 haemorrhage, as the urgency of treatment precludes the time required to obtain the result.[5][9]
Result
<20 mmol/L (<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.
It is helpful in diagnosing an aetiology for acute renal failure. A low value (<1%) indicates that most of the sodium delivered to the kidney is re-absorbed, as is seen in volume depletion and prerenal azotaemia. A value >1% indicates that the kidney is not avidly re-absorbing sodium, either because of diuretic therapy or due to an intrinsic renal defect, such as acute tubular necrosis.
Result
<1%
random urine chloride
Test
Result is usually similar to urine sodium, because chloride is re-absorbed 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 mmol/L [mEq/L]) is a much more reliable indicator of hypovolaemia.[5][9]
Result
<20 mmol/L (<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, hypokalaemia, hypercalcaemia, or central or nephrogenic diabetes insipidus.
Result
>450 mmol/kg (>450 mOsm/kg)
rectal examination and faecal occult blood test
Test
Rectal examination may reveal haematochezia (lower gastrointestinal [GI] bleed) or melaena (upper GI bleed). Faecal occult blood testing may confirm more chronic GI blood loss.
Result
positive with GI blood loss
Investigations to consider
urine urea/fractional excretion of urea (FE urea)
Test
For patients on diuretics. 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.
A low value is seen in prerenal azotaemia or acute renal failure from volume depletion. The test can be helpful in confirming that volume depletion is present in a patient taking diuretics. In this case, the FE urea may be low even if the FENa has been increased by the diuretic drug. Otherwise, this index does not add much to information obtained from FENa.
Result
<35%
arterial blood gases
Test
May be obtained to help assess the acid-base status of the patient. This is important as low bicarbonate levels may occur with prolonged diarrhoea, or high levels of bicarbonate with prolonged vomiting. Hypotension and effective volume depletion from shock often cause lactic acidosis.
Result
metabolic acidosis or alkalosis
nasogastric lavage
Test
Should be the initial manoeuvre 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 aetiology in cases of severe diarrhoea.
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 aetiology.
Result
intra-abdominal and/or intra-luminal fluid may be seen
abdominal CT scan
Test
Depending on clinical scenario, can provide evidence for third-space sequestration of fluid due to ascites or intestinal obstruction, or provide evidence of intra-abdominal trauma and bleeding.
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 colour, and urinary osmolality have low diagnostic accuracy.[23] 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. A recent study compared the utility of saliva osmolality in dehydration and volume depletion in older patients. Saliva osmolality had better diagnostic accuracy than urinary markers when compared against the reference test for impending dehydration (plasma osmolality >295 mOsm/kg) and volume depletion (urea/creatinine ratio >20 in the absence of hypertonicity).[24] Therefore, this may be more suitable as a test in the future.
Result
> 100 mmol/kg
point-of-care magnetic relaxometry
Test
Preliminary data derived from patients with end-stage renal disease who regularly receive haemodialysis suggests that a portable nuclear magnetic resonance device can accurately assess hydration status at the bedside.[17]
Result
expanded muscle extracellular fluid (ECF) space indicative of fluid build-up; decrease in muscle ECF evident upon fluid removal
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