During an ACLS event, blood tests should be obtained as early as possible, ideally immediately when the code team arrives, without delaying or interrupting chest compressions. The primary purpose is to identify reversible causes of the arrest that can be treated in real time. A second critical window for lab work opens the moment a patient achieves return of spontaneous circulation (ROSC), when blood tests guide post-resuscitation stabilization.
Why Labs Matter During Cardiac Arrest
The ACLS algorithm is built around identifying and correcting reversible causes of cardiac arrest, organized into the familiar “H’s and T’s” mnemonic. Several of these causes can only be confirmed through blood testing: abnormal potassium levels (hypokalemia or hyperkalemia), severe acidosis, hypoglycemia, and certain toxin ingestions. Without lab data, the resuscitation team is essentially guessing at which reversible factor might be driving the arrest.
In well-organized code teams, a dedicated lab role is assigned specifically to draw blood immediately upon arrival, send it to the lab as a stat order, and relay results back to the team leader as quickly as possible. This happens in parallel with CPR, defibrillation, and medication administration. The person drawing labs should never be the person doing compressions or managing the airway.
Intra-Arrest Blood Gas Analysis
An arterial blood gas (ABG) drawn during active CPR is one of the most useful tests available mid-resuscitation. It reveals blood pH, oxygen levels, and carbon dioxide levels, all of which help the team understand what’s happening physiologically. The 2025 AHA guidelines note that blood gas analysis is among the adjuncts that can help optimize an individual resuscitation.
Research on in-hospital cardiac arrests found that patients whose blood pH was above 7.2 during the first 10 minutes of CPR were nearly three times more likely to have a favorable neurological recovery compared to those with a pH at or below that threshold. A pH of 7.2 has been proposed as a practical cutoff for defining severe acidosis during arrest. If the blood gas shows profound acidosis driven by high carbon dioxide, the team knows ventilation needs to be optimized. If it shows metabolic acidosis instead, the cause may be something like prolonged low blood flow, kidney failure, or a toxic ingestion.
Small studies have also found that oxygen and carbon dioxide levels on arterial blood gas can help predict whether a patient will achieve ROSC, giving the team leader additional information to guide decision-making as the resuscitation progresses.
Electrolytes: Potassium Is the Priority
Potassium abnormalities are among the most treatable causes of cardiac arrest, but they require a lab value to confirm. Normal serum potassium falls between 3.5 and 5.0 mEq/L. Levels above that range (hyperkalemia) can cause fatal rhythm disturbances, and levels below 2.5 mEq/L are considered severe enough to trigger dangerous arrhythmias on their own.
When hyperkalemia is suspected or confirmed during a code, the treatment shifts immediately to include calcium (to stabilize the heart’s electrical activity) and sodium bicarbonate. For hypokalemia with active arrhythmias, rapid potassium replacement becomes a priority. Neither treatment should be given blindly if lab confirmation is available within a reasonable window, which is why getting blood drawn early matters so much.
Point-of-care testing (POCT), using bedside devices that return results in minutes rather than the 20 to 40 minutes a central lab might take, plays a major role here. A study of out-of-hospital cardiac arrest patients brought to the emergency department found that POCT was used in about 55% of cases. Every single patient tested had at least one abnormal result, and in 91% of those patients the results led directly to a change in treatment. That’s a striking number: nearly all point-of-care results during a code altered what the team did next.
Glucose Levels During and After Arrest
Blood glucose is fast and simple to check at the bedside and should be obtained early in any resuscitation. Hypoglycemia is a reversible cause of altered consciousness and can contribute to cardiac arrest, particularly in patients with diabetes. A fingerstick glucose takes seconds and doesn’t require a venous blood draw.
After ROSC, glucose management becomes a specific focus of post-resuscitation care. The AHA recommends maintaining blood glucose between 144 and 180 mg/dL. Tighter control (targeting 80 to 110 mg/dL) increases the risk of dangerously low blood sugar caused by treatment itself, while allowing levels above 180 to 200 mg/dL worsens outcomes. Frequent glucose monitoring in the hours after ROSC is standard.
Post-ROSC Lab Work
Once a patient regains a pulse, a broader set of blood tests becomes essential. This is the second major window for lab draws in an ACLS event, and it’s just as important as the intra-arrest labs. The goals shift from “find the cause of the arrest” to “stabilize the patient and assess organ damage.”
A complete metabolic panel helps evaluate kidney function, electrolyte balance, and acid-base status in a more controlled setting. Repeat blood gas analysis confirms whether oxygenation and ventilation targets are being met. Lactate levels indicate how long tissues were deprived of adequate blood flow.
Cardiac biomarkers like troponin help determine whether a heart attack caused or resulted from the arrest. Troponin levels begin rising 3 to 4 hours after heart muscle damage starts, so a single value drawn immediately after ROSC may not tell the full story. Serial testing over the following hours is far more reliable, because clinicians need to see a rising or falling pattern rather than relying on one number.
Neurological Prognostication After 72 Hours
For patients who remain unconscious after cardiac arrest, blood tests drawn at a later time point can help predict neurological outcomes. The 2025 AHA guidelines note that elevated levels of certain brain injury markers in the blood, measured within 72 hours of the arrest, can support (though not single-handedly determine) a prognosis of poor neurological recovery. These markers are always used alongside other assessment tools like brain imaging and neurological exams, never in isolation.
Timing Summary for ACLS Lab Draws
- Immediately at code onset: Blood gas, electrolytes (especially potassium), glucose, and a toxicology screen if poisoning is suspected. Use point-of-care devices when available for the fastest turnaround.
- During active CPR: Results from initial draws are relayed to the team leader to guide real-time treatment decisions. Repeat blood gas if the resuscitation is prolonged.
- Immediately after ROSC: Full metabolic panel, repeat blood gas, lactate, glucose, and initial cardiac biomarkers. These guide the transition to post-resuscitation care.
- Hours after ROSC: Serial troponin levels, repeat electrolytes, and ongoing glucose monitoring to track trends and adjust treatment.
- 24 to 72 hours post-arrest: Brain injury biomarkers if the patient remains comatose, used alongside other prognostic tools.
The consistent principle across all of these windows is that blood tests should never delay high-quality CPR or defibrillation. They are drawn by a designated team member working alongside the resuscitation, not instead of it.