Blood Tests for Suspected Shock in Children Without Lactate Measurement
When lactate measurement is unavailable in a child with suspected shock, obtain serial blood gases (for pH, base deficit, and bicarbonate), central venous oxygen saturation (ScvO2), complete blood count, electrolytes, renal function tests, and blood cultures to assess hypoperfusion and guide early therapy.
Primary Laboratory Assessment
While the Surviving Sepsis Campaign guidelines specifically recommend using trends in blood lactate levels in addition to clinical assessment to guide resuscitation 1, when lactate is unavailable, you must rely on alternative markers of tissue hypoperfusion and organ dysfunction.
Core Blood Tests to Obtain:
Markers of Tissue Hypoperfusion (when lactate unavailable):
- Arterial or venous blood gas: Provides pH, base deficit, and bicarbonate levels—these reflect metabolic acidosis from tissue hypoperfusion
- Central venous oxygen saturation (ScvO2): The guidelines explicitly mention ScvO2 as an advanced hemodynamic variable to guide resuscitation 1
- Arteriovenous CO2 gap: Can accurately reflect oxygen delivery when ScvO2 is normal, as tissue hypoxia may exist even with normal ScvO2 2
Essential Supporting Laboratory Tests:
- Complete blood count: Assess for anemia, thrombocytopenia, and leukocyte abnormalities
- Electrolytes and renal function: Identify metabolic derangements and acute kidney injury
- Blood cultures: Mandatory before antibiotics in suspected septic shock
- Coagulation studies: Assess for disseminated intravascular coagulation
- Liver function tests: Evaluate hepatic dysfunction from hypoperfusion
Critical Interpretation Points
Base deficit and pH serve as reasonable surrogates for lactate in assessing metabolic acidosis from tissue hypoperfusion. A base deficit >5 mEq/L or pH <7.30 suggests significant metabolic acidosis requiring aggressive resuscitation.
ScvO2 monitoring is specifically endorsed by the guidelines as an advanced hemodynamic variable 1. Target ScvO2 >70% during resuscitation. However, recognize that normal ScvO2 does not exclude tissue hypoxia—this is where CO2 gap becomes valuable 2.
Serial Monitoring Strategy
The guidelines emphasize that frequent reassessment is crucial 1. Obtain repeat blood gases every 1-2 hours during active resuscitation to track:
- Improvement or worsening of base deficit
- Resolution of metabolic acidosis
- Response to fluid and vasoactive therapy
Recent evidence confirms that lactate kinetics (trends over time) correlate better with outcomes than single measurements 3, 4, 5. Apply this same principle to base deficit—serial measurements showing improvement indicate adequate resuscitation.
Common Pitfalls to Avoid
Do not rely on clinical signs alone. The guidelines explicitly state not to use bedside clinical signs in isolation 1, and research confirms poor correlation between clinical assessment and invasive hemodynamic parameters 6, 7. While clinical signs like feeble pulses, tachycardia, and prolonged capillary refill time have good diagnostic accuracy 8, they must be combined with laboratory markers.
Do not assume normal ScvO2 excludes shock. Tissue hypoxia can persist despite normal ScvO2 due to microcirculatory dysfunction, mitochondrial impairment, and shunting 2, 4, 5.
Do not delay resuscitation waiting for laboratory results. Begin fluid resuscitation based on clinical assessment while obtaining blood tests. The guidelines recommend 40-60 mL/kg fluid boluses in the first hour in settings with intensive care availability 1.
Integration with Clinical Assessment
Combine laboratory findings with clinical markers of cardiac output: heart rate, blood pressure, capillary refill time, level of consciousness, and urine output 1. The presence of altered sensorium and requirement for multiple inotropes predict poor outcomes 9, making early laboratory assessment even more critical in these patients.
Monitor for fluid overload during resuscitation by assessing for pulmonary edema or new/worsening hepatomegaly 1, as this should limit further fluid boluses regardless of laboratory values.