Highlights
- Point-of-care direct potentiometry (DP) and central laboratory indirect potentiometry (IP) are not interchangeable for sodium (Na+) and potassium (K+) measurements in the ICU.
- Prospective data reveals a 95% limit of agreement for Na+ of 10.48 mmol/L, which exceeds clinically acceptable thresholds for acute management.
- Approximately 10% of electrolyte measurements result in discordant clinical classifications (e.g., normal vs. abnormal) when comparing the two methods.
- A significant knowledge gap exists among clinicians, with only 31.1% of surveyed physicians aware of the analytical biases inherent in indirect potentiometry.
Background
Sodium and potassium ions are the most frequently monitored electrolytes in the Intensive Care Unit (ICU). Accurate measurement is critical, as rapid shifts or incorrect management of these ions can lead to severe neurological complications, cardiac arrhythmias, and increased mortality. In modern clinical practice, two distinct technologies are utilized: point-of-care (POC) blood gas analyzers using Direct Potentiometry (DP) and central laboratory automated analyzers using Indirect Potentiometry (IP).
While both methods rely on ion-selective electrodes (ISE), they differ fundamentally in sample preparation. DP measures the ion activity in undiluted whole blood, reflecting the concentration in the plasma water phase. Conversely, IP involves a dilution step of the plasma sample before measurement. This dilution assumes that plasma is composed of approximately 93% water and 7% solids (lipids and proteins). In critically ill patients, this assumption frequently fails due to common conditions such as hypoalbuminemia, hyperlipidemia, or paraproteinemia, leading to clinically significant analytical biases.
Key Content
The Analytical Divide: Direct vs. Indirect Potentiometry
The core of the discrepancy lies in the “electrolyte exclusion effect.” Indirect potentiometry calculates ion concentration based on the total volume of the diluted sample. If the non-aqueous fraction of the plasma (proteins and lipids) is significantly higher or lower than the physiological norm, the calculated concentration will be erroneously low (pseudohyponatremia in hyperproteinemia) or high (pseudohypernatremia in hypoproteinemia). Direct potentiometry, by measuring the aqueous phase directly without dilution, remains unaffected by these variations in plasma solid content, making it theoretically superior in the heterogenous ICU population.
Findings from the Nakhil et al. Study (2026)
A prospective study conducted by Nakhil and colleagues (PMID: 41823506) in a tertiary hospital ICU provides a contemporary quantification of these discrepancies. Analyzing 501 paired measurements, the researchers observed only moderate agreement between DP and IP. The Lin’s concordance correlation coefficient (CCC) was 0.90 for Na+ and 0.93 for K+. More alarmingly, the 95% limit of agreement (LoA) for Na+ was 10.48 mmol/L. Given that the entire normal range for sodium is approximately 10 mmol/L, an error margin of this magnitude can lead to diametrically opposed clinical decisions.
The study specifically looked at clinical classification divergence. In 10% of cases, the two methods disagreed on whether a patient was hyponatremic, normonatremic, or hypernatremic. Similar rates of discordance were found for potassium, often influenced by hemolysis during sample transport to the central laboratory—a factor that POC testing bypasses.
The Physician Awareness Gap
Beyond the analytical data, the study investigated the human element. Among 103 responding physicians, knowledge regarding these laboratory techniques was surprisingly low:
- Sodium Awareness: Only 31.1% understood the bias associated with IP in the context of altered protein levels.
- Potassium Reliability: 45.6% of clinicians believed DP was as reliable or more reliable than IP for potassium, despite the fact that central labs are often considered the “gold standard” despite transport-induced hemolysis risks.
This lack of awareness suggests that clinicians may be making high-stakes decisions (such as initiating hypertonic saline or aggressive potassium replacement) based on values that vary solely due to the measurement technique rather than the patient’s actual physiological state.
Impact of Proteinemia and Hemolysis
The study confirmed that protein levels significantly correlate with the Na+ discrepancy. In patients with severe hypoalbuminemia (common in sepsis and malnutrition), IP tends to overestimate sodium compared to DP. For potassium, the primary driver of discrepancy was pre-analytical: the time delay and physical stress of transporting samples to a central lab increased the risk of hemolysis, which artificially elevates IP potassium levels. POC testing, performed immediately at the bedside, minimizes this risk.
Expert Commentary
The lack of interchangeability between DP and IP is a well-known phenomenon in clinical chemistry, yet it remains underappreciated at the bedside. The findings of Nakhil et al. underscore a critical safety concern in the ICU. The 10% discrepancy rate in clinical classification is not merely a statistical nuance; it represents potential medical errors. For instance, a patient might be classified as hyponatremic by a central lab (IP) while being normonatremic by a blood gas analyzer (DP) due to hyperproteinemia.
In the context of evidence-based guidelines, many experts suggest that Direct Potentiometry (POC) should be the preferred method for sodium management in the ICU, particularly when protein levels are abnormal. However, the central laboratory often provides better precision for longitudinal tracking. The key is consistency. Clinicians must choose one method for trending and be aware of the “physiological noise” introduced when switching between POC and central lab results.
Furthermore, the survey results highlight an urgent need for education. Medical curricula and residency training must emphasize the technological limitations of laboratory medicine. A “lab value” is not an absolute truth but an estimate generated by a specific methodology with specific vulnerabilities.
Conclusion
The study by Nakhil et al. serves as a vital reminder that in the high-acuity environment of the ICU, the method of measurement matters as much as the result itself. IP and DP are not interchangeable, and the current 10% discordance rate in electrolyte classification poses a risk to patient safety. To improve the quality of care, healthcare systems should prioritize physician education on analytical biases and establish clear protocols on which measurement method to prioritize in specific clinical scenarios, such as the management of severe dysnatremias or suspected hemolysis-related hyperkalemia.
References
- Nakhil N, Najem S, Driss R, Chaabouni T, Dufour N. Discrepancies Between Point of Care and Central Laboratory Sodium and Potassium Measurements in ICU: Analytical Biases and Physician Awareness. Critical care medicine. 2026-03-13. PMID: 41823506.
- Chacko B, Peter JV, Patole S, Fleming JJ, Selvakumar R. Electrolytes by an indirect ion-selective electrode analyzer: a source of error in patients with hypoproteinemia. Crit Care Resusc. 2011;13(3):149-53. PMID: 21880002.
- Fortgens P, Pillay TS. Pseudohyponatremia revisited: a modern-day pitfall due to hyperproteinemia. Arch Pathol Lab Med. 2011;135(12):1534-9. PMID: 22129181.
