What labs should be checked in a patient suspected of dehydration and rhabdomyolysis, particularly in vulnerable populations such as the elderly, athletes, or those with pre-existing kidney disease?

Laboratory Evaluation for Dehydration and Rhabdomyolysis

In patients suspected of dehydration and rhabdomyolysis, immediately obtain serum osmolality (or calculate it if unavailable), creatine kinase (CK), serum creatinine, complete metabolic panel including sodium, potassium, calcium, phosphate, bicarbonate, and urinalysis for myoglobinuria. 1, 2

Essential Initial Laboratory Tests

For Dehydration Assessment

• Serum/plasma osmolality is the gold standard for determining hydration status, with values >300 mOsm/kg indicating dehydration 1
• If direct osmolality measurement is unavailable, calculate serum osmolarity using: osmolarity = 1.86 × (Na⁺ + K⁺) + 1.15 × glucose + urea + 14 (all in mmol/L), with action threshold >295 mmol/L 1
• Measure serum sodium, potassium, glucose, and urea as these are the key components contributing to osmolality 1
• Corrected serum sodium for hyperglycemia should be calculated (add 1.6 mEq to sodium value for each 100 mg/dL glucose >100 mg/dL) to accurately assess true sodium status, particularly critical in diabetic patients 1
• Blood urea nitrogen (BUN) and creatinine to assess renal function and pre-renal azotemia 3

For Rhabdomyolysis Diagnosis

• Serum creatine kinase (CK) level >1000 U/L or at least 5× the upper limit of normal confirms rhabdomyolysis 2
• Serum myoglobin should be measured, though CK is more reliable for diagnosis 4, 2
• Urinalysis for myoglobinuria (dipstick positive for blood without RBCs on microscopy indicates myoglobin) 2, 5
• Complete metabolic panel including serum creatinine, BUN, sodium, potassium, calcium, phosphate, magnesium, chloride, and bicarbonate 3, 2
• Serum calcium and phosphate are critical as hypocalcemia and hyperphosphatemia are common complications 3
• Arterial blood gas if acidosis is suspected 3
• Complete blood count to assess for anemia or infection 3

Monitoring Frequency During Treatment

• Electrolytes should be checked every 2-4 hours during initial treatment of severe dehydration or rhabdomyolysis 1
• Blood glucose should be monitored every 1-2 hours until stable in cases of hyperglycemia-related dehydration 1
• Calculate effective serum osmolality regularly to guide fluid management and prevent cerebral edema during correction 1
• Serial CK measurements every 6-12 hours to track progression or resolution of rhabdomyolysis 2
• Serum creatinine and urine output should be monitored closely to detect acute kidney injury early 3, 4

Risk Stratification

• McMahon score ≥6 on admission predicts acute kidney injury requiring renal replacement therapy in rhabdomyolysis patients 2
• In crush injury patients, point-of-care devices (e.g., iStat) can provide rapid creatinine and potassium results within minutes when standard laboratory infrastructure is unavailable 3

Tests NOT Recommended

• Do NOT use urinary indices (color, specific gravity, osmolality) to assess hydration status in older adults as they are unreliable 1
• Do NOT use bioelectrical impedance analysis (BIA) to assess hydration status as it has not been shown to be consistently diagnostic 1
• Avoid relying solely on clinical signs (skin turgor, mouth dryness) without laboratory confirmation, as these are subtle and unreliable, especially in older adults 1, 6

Special Considerations for Vulnerable Populations

Elderly Patients

• All older persons should be screened for dehydration when they contact the healthcare system, if clinical condition changes unexpectedly, or when malnourished 1, 6
• Older adults have blunted thirst sensation and reduced renal concentrating ability, making laboratory assessment essential 6
• Serum osmolality >300 mOsm/kg in elderly patients with impaired renal function indicates dehydration, not SIADH 7

Athletes and Exertional Cases

• In athletes with severe cramping and suspected exertional rhabdomyolysis, immediate CK testing is essential as levels can rise rapidly 8
• Dehydration commonly precedes exertional rhabdomyolysis in athletes, making combined assessment critical 8

Patients with Pre-existing Kidney Disease

• Baseline creatinine is essential to distinguish acute kidney injury from chronic kidney disease 3
• In CKD patients, measure Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, blood urea, creatinine, and bicarbonate levels as part of preoperative or acute assessment 3
• Serum potassium monitoring is critical in CKD patients receiving diuretics, ACE inhibitors, ARBs, or MRAs 3

Common Pitfalls to Avoid

• Failing to correct serum sodium for hyperglycemia in diabetic patients can mask true sodium status and lead to inappropriate fluid management 1
• Relying on normal-sized kidneys on imaging to exclude CKD, as renal size is initially preserved in diabetic nephropathy 3
• Confusing elevated creatinine from ACE inhibitors/ARBs (up to 30% increase acceptable) with acute kidney injury in the absence of volume depletion 3
• Delaying laboratory assessment in admitted patients—rapid testing is essential to avoid delays in initial therapy 1
• In rhabdomyolysis, potassium-containing fluids (Lactated Ringer's, Hartmann's solution) must be avoided as potassium levels may increase markedly even with intact renal function 3