Point-of-Care Lactate and Electrolytes

Point-of-care (POC) lactate and electrolyte testing uses handheld meters (lactate strips with amperometric lactate oxidase electrodes) and cartridge based blood gas analysers (ion selective electrodes for sodium, potassium, chloride, ionised calcium and sometimes magnesium) to give results in 30 seconds to 2 minutes from 0.1 to 0.2 mL of capillary, venous or arterial whole blood.

Handheld lactate meters correlate closely with laboratory lactate (correlation 0.9 or more) in the range 0.5 to 10 mmol/L, with lower accuracy over 15 mmol/L and in shock with poor peripheral perfusion. Cartridge analysers match laboratory values for sodium, potassium and ionised calcium within accepted limits.

Lactate is produced by anaerobic glycolysis, and rises in tissue hypoperfusion (type A) and in conditions with impaired clearance or increased glycolysis without hypoxia (type B: liver failure, metformin, thiamine deficiency, malignancy, adrenaline, salbutamol, seizures, HIV drugs).

Hyperlactataemia over 2 mmol/L identifies patients with sepsis at risk of death, and levels over 4 mmol/L carry mortality of 30 to 40% in sepsis. Dyskalaemia and dysnatraemia are common in emergencies (potassium disturbance occurs in 10 to 20% of hospital admissions), and rapid results at the bedside change management during resuscitation, cardiac arrest, renal failure and diabetic emergencies.

Indications. Suspected sepsis or septic shock. Shock of any cause, major trauma and haemorrhage. Cardiac arrest and post-resuscitation care. Severe malaria and severe malnutrition. Acute kidney injury, suspected hyperkalaemia (ECG changes, weakness), dialysis patients. Diabetic ketoacidosis and hyperosmolar states. Poisoning. Vomiting and diarrhoea with dehydration.

Obstetric emergencies (haemorrhage, pre-eclampsia, sepsis). Neonates with respiratory distress or suspected sepsis. Monitoring response to resuscitation. Pre-transfer assessment in remote clinics.

Specimen. Whole blood 0.1 to 0.2 mL, capillary (fingertip or heel) for lactate strips, or venous or arterial blood in a heparinised syringe (balanced heparin) for cartridges. Avoid prolonged tourniquet (stasis raises lactate and potassium), fist clenching, sampling from an arm with infusion running, and delay (lactate rises in whole blood at room temperature). Test within 10 minutes. Record the site, time, oxygen therapy and clinical context.

Reference values. Lactate 0.5 to 2.0 mmol/L (arterial 0.5 to 1.6). Sodium 135 to 145 mmol/L. Potassium 3.5 to 5.0 mmol/L. Chloride 98 to 107 mmol/L. Ionised calcium 1.15 to 1.30 mmol/L. Bicarbonate 22 to 28 mmol/L. Anion gap 8 to 16 mmol/L.

Interpretation.

• Lactate 2 to 4 mmol/L: hyperlactataemia, with repeat testing and search for cause. Over 4 mmol/L: severe, with high mortality, and treated as septic shock if infection is suspected.
• Failure of lactate to fall by at least 10% in 2 hours (or normalise within 6 hours) indicates poor response to resuscitation and worse outcome.
• Lactate with normal pH and normal perfusion: type B causes, such as metformin, adrenaline, salbutamol, liver disease, seizures, or malignancy, and drugs (linezolid, nucleoside reverse transcriptase inhibitors).
• D-lactate (short gut, bacterial overgrowth) is not detected by routine assays and is suspected in unexplained encephalopathy and acidosis.
• Potassium over 5.5 mmol/L: hyperkalaemia (renal failure, acidosis, drugs, tissue breakdown); over 6.5 mmol/L, a medical emergency. Whole blood potassium avoids in vitro haemolysis seen in laboratory serum.
• Potassium under 3.5 mmol/L: hypokalaemia (diuretics, vomiting, diarrhoea, insulin, hypomagnesaemia, hyperaldosteronism), and under 2.5 mmol/L is life threatening.
• Sodium under 135 mmol/L: hyponatraemia, assessed with osmolality, volume status and urine sodium. Over 145 mmol/L: hypernatraemia (water loss, diabetes insipidus, hypertonic fluids).
• Ionised calcium under 1.1 mmol/L: hypocalcaemia (massive transfusion with citrate, pancreatitis, sepsis, renal failure). Over 1.35 mmol/L: hypercalcaemia (malignancy, hyperparathyroidism).
• Chloride and anion gap help to separate high anion gap acidosis from hyperchloraemic acidosis after large saline volumes.

Critical values and findings needing immediate action.

• Lactate over 4 mmol/L, or over 2 mmol/L with hypotension: sepsis bundle with fluid resuscitation, antibiotics within an hour, source control, vasopressors, and repeat lactate in 2 hours.
• Potassium over 6.5 mmol/L, or over 5.5 mmol/L with ECG changes: intravenous calcium, insulin with glucose, salbutamol, and treatment of cause; dialysis in refractory cases.
• Potassium under 2.5 mmol/L: intravenous potassium with cardiac monitoring, and magnesium replacement.
• Sodium under 120 mmol/L with seizures or coma: hypertonic saline as the clinical picture requires. Sodium over 160 mmol/L: slow correction of free water deficit.
• Ionised calcium under 0.9 mmol/L, or with tetany, seizures or arrhythmia: intravenous calcium.
• Lactate over 10 mmol/L: profound shock, mesenteric ischaemia, or toxic cause (metformin), requiring critical care and renal support.

Interferences and pitfalls.

• Poor peripheral perfusion, cold fingers and shock reduce the accuracy of capillary samples; venous or arterial samples are preferred.
• Prolonged tourniquet time, fist pumping and delayed analysis raise lactate, and delay or cold storage raises potassium by leakage from cells.
• Haemolysis and leucocytosis (more than 100 times 10 to the power 9/L) may influence results; high platelet counts cause pseudohyperkalaemia in serum but less in whole blood.
• Interfering substances for strips: ascorbic acid, paracetamol, N-acetylcysteine, glycolate (ethylene glycol poisoning causes falsely high lactate on some analysers), and haematocrit extremes.
• Hyperproteinaemia and hyperlipidaemia cause pseudohyponatraemia with indirect ion selective electrodes (laboratory analysers) but not with direct (whole blood blood gas analysers), which read the true sodium.
• Ionised calcium is pH-dependent (falls about 0.05 mmol/L for each 0.1 rise in pH), and heparin binds calcium; balanced heparin cartridges are needed.
• Cartridges are sensitive to storage temperature, and expired or frozen cartridges give errors; quality control at defined intervals is required.
• Lactate alone does not diagnose sepsis, and a normal lactate does not exclude sepsis or shock (especially early, in young patients, and in cirrhosis).
• Results from different analysers (laboratory, blood gas, handheld) may differ, and trends should come from the same device.
• Regional differences in availability of cartridges and meters limit access in low resource facilities.

Special groups.

• Children and neonates: lactate over 4 mmol/L in severe malaria predicts death, and in neonates lactate is higher in the first hours; microsamples (heel prick) are used.
• Pregnancy: lactate rises in labour, and levels over 2.5 mmol/L with sepsis or haemorrhage warrant action.
• Renal failure and dialysis: potassium monitoring before and after dialysis, with risk of rebound.
• Cirrhosis: reduced lactate clearance and baseline hyperlactataemia.
• Athletes and exercise: transient lactate increase to over 10 mmol/L, which clears within an hour.
• Regions: POC lactate has improved triage of sepsis, severe malaria and obstetric emergencies in low resource hospitals, where laboratory turnaround is long.

Follow up tests. Venous or arterial blood gas with pH and bicarbonate, laboratory electrolytes, urea and creatinine, magnesium and phosphate, glucose and ketones, serum osmolality, repeat lactate at 2 hours, blood cultures, procalcitonin, ECG, liver function, toxicology tests (metformin, salicylate, toxic alcohols), cortisol if refractory shock, imaging for source control, and urine output monitoring.