Reliability of Point-of-Care Venous Blood Gas Testing for Potassium and Sodium Compared to Central Laboratory Results in Emergency Patients: A Clinical Audit

Whole-blood POCT analyzers tend to report slightly elevated potassium levels compared to serum-based laboratory systems due to differences in sample handling and electrode sensitivity.

“This minor overestimation bias is consistent with earlier work by Chhapola et al. [3] and Jain et al. [4], who observed that whole-blood POCT analyzers tend to report slightly elevated K⁺ levels compared to serum-based lab systems due to differences in sample handling and electrode sensitivity.”

• cites Chhapola et al. [3] Chhapola V, Kanwal SK, Sharma R, Kumar V. A comparative study on reliability of point of care sodium and potassium estimation in a pediatric intensive care unit. Indian J Pediatr. 2013, 80:731-5. 10.1007/s12098-013-0977-z sourceNot checked — no full text of the source
• cites Jain et al. [4] Jain A, Subhan I, Joshi M. Comparison of the point-of-care blood gas analyzer versus the laboratory auto-analyzer for the measurement of electrolytes. Int J Emerg Med. 2009, 2:117-20. 10.1007/s12245-009-0091-1 sourceDisputed

  The Results report the opposite direction: mean potassium was 3.7 mmol/l on the blood gas analyzer versus 3.9 mmol/l on the laboratory auto-analyzer, and 71.5% of blood gas readings were lower. The Introduction describes differences in sample type and electrode technology, but does not attribute an upward potassium bias to them. The paper’s conclusion says the potassium measurements did not differ significantly. (LLM, conf=0.90)

  Checked by a reasoning model — may be wrong.

  Experimental — accuracy not yet measured

Delays in identifying electrolyte abnormalities in critically ill patients can significantly increase the risk of morbidity and mortality.

“In critically ill patients, delays in identifying electrolyte abnormalities can significantly increase the risk of morbidity and mortality, making rapid and accurate testing essential [1].”

• cites [1] Balcı AK, Koksal O, Kose A, Armagan E, Ozdemir F, Inal T, Oner N. General characteristics of patients with electrolyte imbalance admitted to emergency department. World J Emerg Med. 2013, 4:113-6. 10.5847/wjem.j.issn.1920-8642.2013.02.005 sourcePartly supported

  The Discussion (page unavailable) says sodium imbalances are associated with increased mortality in patients needing intensive care and notes that hypomagnesemia is often overlooked. It does not show that delays in identifying electrolyte abnormalities increase morbidity or mortality, or establish a need for rapid testing. (LLM, conf=0.84)

  Experimental — accuracy not yet measured

Multiple factors contribute to discrepancies between POCT and laboratory values: variation in sample matrix, pre-analytical handling, device calibration intervals, and analytical methodology.

“Multiple factors contribute to discrepancies between POCT and laboratory values: variation in sample matrix (whole blood vs. serum/plasma), pre-analytical handling, device calibration intervals, and analytical methodology [4,5].”

• cites [4] Jain A, Subhan I, Joshi M. Comparison of the point-of-care blood gas analyzer versus the laboratory auto-analyzer for the measurement of electrolytes. Int J Emerg Med. 2009, 2:117-20. 10.1007/s12245-009-0091-1 sourcePartly supported

  The Introduction (Table 1; page not provided) contrasts whole blood with serum, direct with indirect electrode methods, and different sample dilutions; it also explains how heparin dilution can lower measured electrolyte values. Materials and methods (page not provided) describes sample handling and says both analyzers were calibrated before each measurement cycle. It does not identify variation in device calibration intervals as a cause of discrepancies. (LLM, conf=0.88)

  Experimental — accuracy not yet measured
• cites [5] Nigro M, Valli G, Marchionne ML, et al.. Is there a risk of misinterpretation of potassium concentration from undetectable hemolysis using a POCT blood gas analyzer in the emergency department?. Medicina (Kaunas). 2022, 59:66. 10.3390/medicina59010066 sourcePartly supported

  The Introduction and Discussion support pre-analytical handling as a source of error: collection, transport, storage, and hemolysis can alter measured potassium. The Introduction also notes that central laboratories routinely check for hemolysis whereas blood-gas POCT systems do not. Our copy does not identify sample-matrix variation or device calibration intervals as causes of POCT–laboratory discrepancies, or establish analytical methodology more generally as a cause. (LLM, conf=0.87)

  Experimental — accuracy not yet measured

POCT often underestimates sodium concentrations by 1-4 mmol/L due to methodological differences in ion-selective electrodes.

“Na⁺ measurement discrepancies are also well documented, with several researchers noting that POCT often underestimates Na⁺ concentrations by 1-4 mmol/L due to methodological differences in ion-selective electrodes [9,10].”

• cites [9] Goldstein L, Wells M, Vincent-Lambert C. A randomized controlled trial to assess the impact of upfront point-of-care testing on emergency department treatment time. Am J Clin Pathol. 2018, 150:224-34. 10.1093/ajcp/aqy042 sourceCould not verify

  The 'Abbott Point of Care i-STAT System' section (page not provided) lists sodium among the tests performed, but gives no sodium comparison with laboratory results. Searches for sodium underestimation, a 1–4 mmol/L difference, and ion-selective electrode methods found no passage addressing the claimed discrepancy or its cause. (LLM, conf=0.76)

  Experimental — accuracy not yet measured
• cites [10] Nichols JH, Alter D, Chen Y, et el.. AACC guidance document on management of point-of-care testing. J Appl Lab Med. 2020, 1-20. sourceNot checked — no full text of the source

Sodium underestimation patterns are supported by studies attributing discrepancies to differences in ion-selective electrode calibration, matrix effects, and ambient temperature and humidity.

“These patterns are supported by studies such as Kapoor et al. [7] and Allardet-Servent et al. [8], who attribute these discrepancies to differences in ISE calibration, matrix effects, and the influence of ambient temperature and humidity on POCT devices.”

• cites Kapoor et al. [7] Kapoor D, Srivastava M, Singh P. Point of care blood gases with electrolytes and lactates in adult emergencies. Int J Crit Illn Inj Sci. 2014, 4:216-22. 10.4103/2229-5151.141411 sourcePartly supported

  In the Electrolytes—Sodium, Potassium, Chloride section (page unavailable), the review reports sodium underestimation in a pediatric ICU study and says electrolyte discrepancies may reflect device calibrators, sample differences, and direct versus indirect ion-selective electrodes affected by proteins. It does not attribute the discrepancies to ambient temperature or humidity, and it presents the proposed causes cautiously rather than as established explanations. (LLM, conf=0.87)

  Experimental — accuracy not yet measured
• cites Allardet-Servent et al. [8] Allardet-Servent J, Lebsir M, Dubroca C, et al.. Point-of-care versus central laboratory measurements of hemoglobin, hematocrit, glucose, bicarbonate and electrolytes: a prospective observational study in critically ill patients. PLoS One. 2017, 12:e0169593. 10.1371/journal.pone.0169593 sourcePartly supported

  The Discussion (page not provided) attributes the sodium difference to direct versus indirect ISE measurement and the effect of protein content on dilution; it reports sodium averaging 1.3 mmol/L higher with indirect ISE. This supports a sample-matrix explanation for lower point-of-care readings relative to the laboratory, but the paper does not attribute the discrepancy to ISE calibration or ambient temperature and humidity. (LLM, conf=0.88)

  Experimental — accuracy not yet measured

In this audit, the mean bias between POCT and laboratory sodium values was −1.2 mmol/L.

“The Bland-Altman analysis (Figure 4) showed a mean bias of −1.2 mmol/L, with 95% LoA between −5.4 and +3.0 mmol/L.”

• Also stated in: § Discussion · hedged

  “Na⁺ measurement showed a similarly strong correlation (r = 0.88; p < 0.001), though with a mean negative bias of −1.2 mmol/L, suggesting a tendency of POCT to underestimate Na⁺ levels.”

In this audit, the mean bias between POCT and laboratory potassium values was +0.12 mmol/L.

“Further assessment using Bland-Altman analysis (Figure 2) showed a mean bias of +0.12 mmol/L, with 95% LoA ranging from −0.89 to +1.13 mmol/L.”

• Also stated in: § Discussion · plain

  “Bland-Altman analysis revealed a mean positive bias of +0.12 mmol/L, indicating slight overestimation.”

In this audit, POCT and central laboratory sodium values had a Pearson correlation coefficient of r = 0.88 (p < 0.001).

“The correlation between Na⁺ values from POCT and lab testing was also strong (Figure 3), with a Pearson correlation coefficient of r = 0.88 (p < 0.001)”

• Also stated in: § Discussion · plain

  “Na⁺ measurement showed a similarly strong correlation (r = 0.88; p < 0.001), though with a mean negative bias of −1.2 mmol/L, suggesting a tendency of POCT to underestimate Na⁺ levels.”

In this audit of 120 adult ED patients, POCT and central laboratory potassium values had a Pearson correlation coefficient of r = 0.91 (p < 0.001).

“There was a very strong correlation between POCT and laboratory K⁺ values, with a Pearson correlation coefficient of r = 0.91 (p < 0.001).”

• Also stated in: § Discussion · boosted

  “In this audit, POCT K⁺ values demonstrated a very strong correlation with central laboratory values (r = 0.91; p < 0.001).”

In this audit, POCT mean turnaround time was 2.4 minutes (SD 0.6).

“POCT results were available in an average of 2.4 minutes (standard deviation: 0.6 minutes), whereas central lab results took an average of 12.3 minutes (standard deviation: 3.1 minutes).”

In this audit, central laboratory mean turnaround time was 12.3 minutes (SD 3.1).

“POCT results were available in an average of 2.4 minutes (standard deviation: 0.6 minutes), whereas central lab results took an average of 12.3 minutes (standard deviation: 3.1 minutes).”

In this audit, the 95% limits of agreement for sodium between POCT and laboratory values ranged from −5.4 to +3.0 mmol/L.

“The Bland-Altman analysis (Figure 4) showed a mean bias of −1.2 mmol/L, with 95% LoA between −5.4 and +3.0 mmol/L.”

In this audit, 117 of 120 patients (97.5%) had potassium values within clinically acceptable limits on both POCT and laboratory platforms.

“Importantly, 117 out of 120 patients (97.5%) had K⁺ values within clinically acceptable limits on both platforms.”

In this audit, the 95% limits of agreement for potassium between POCT and laboratory values ranged from −0.89 to +1.13 mmol/L.

“Further assessment using Bland-Altman analysis (Figure 2) showed a mean bias of +0.12 mmol/L, with 95% LoA ranging from −0.89 to +1.13 mmol/L.”

In this audit, 11 patients (9.2%) had sodium values in the low-normal range (135-138 mmol/L) that were underestimated by POCT relative to laboratory measurement.

“Importantly, 11 patients (9.2%) had Na⁺ values in the low-normal range (135-138 mmol/L) that were underestimated by POCT relative to the laboratory measurement.”

• Also stated in: § Discussion · plain

  “Eleven patients (9.2%) had POCT Na⁺ values in the lower end of the normal range (135-138 mmol/L) that were lower than corresponding central lab values.”

In this audit, all 120 patients (100%) had POCT results available immediately at the bedside.

“All 120 patients (100%) had POCT results available immediately at the bedside, allowing for accelerated diagnostic formulation and therapeutic decision-making.”

In this audit, three patients (2.5%) had POCT potassium values above 5.5 mmol/L that were not confirmed by laboratory testing, indicating false-positive hyperkalemia from POCT.

“Despite the overall agreement, three patients (2.5%) showed K⁺ values above 5.5 mmol/L on POCT that were not confirmed by the lab test, indicating false-positive hyperkalemia from POCT.”

• Also stated in: § Discussion · plain

  “While this falls within acceptable clinical tolerance, three out of 120 patients (2.5%) exhibited POCT K⁺ values exceeding 5.5 mmol/L, which were not corroborated by laboratory analysis.”

Spectrum bias may be present because the study cohort comprised only ED patients, who may differ from inpatient or ICU cohorts.

“Moreover, spectrum bias may be present, as ED patients may differ from inpatient or ICU cohorts in disease acuity and electrolyte disturbance profiles [6].”

• Also stated in: § Discussion · hedged

  “Limitations include device-specific analytic range, lack of outcome data, and possible spectrum bias, as the study cohort comprised only ED patients.”

• cites [6] Gavala A, Myrianthefs P. Comparison of point-of-care versus central laboratory measurement of hematocrit, hemoglobin, and electrolyte concentrations. Heart Lung. 2017, 46:246-50. 10.1016/j.hrtlng.2017.04.003 sourceNot checked — no full text of the source

The observed discrepancies were within generally accepted analytical thresholds.

“Although the observed discrepancies were within generally accepted analytical thresholds, defined as ≤ 0.3 mmol/L for K⁺ and ≤ 3 mmol/L for Na⁺, they may still impact management decisions in borderline or clinically sensitive cases.”

These variables may cause systematic bias, especially in high-risk thresholds such as hyperkalemia (>5.5 mmol/L) or severe hyponatremia (<125 mmol/L).

“These variables may cause systematic bias, especially in high-risk thresholds such as hyperkalemia (> 5.5 mmol/L) or severe hyponatremia (< 125 mmol/L), where erroneous values could trigger inappropriate clinical responses [6].”

• cites [6] Gavala A, Myrianthefs P. Comparison of point-of-care versus central laboratory measurement of hematocrit, hemoglobin, and electrolyte concentrations. Heart Lung. 2017, 46:246-50. 10.1016/j.hrtlng.2017.04.003 sourceNot checked — no full text of the source

POCT demonstrated high reliability for K+ and Na+ evaluation in EDs.

“POCT demonstrated high reliability for K⁺ and Na⁺ evaluation in EDs.”

POCT underestimation of sodium in the low-normal range could lead to overdiagnosis of mild hyponatremia.

“This trend could result in overdiagnosis of mild hyponatremia and may influence fluid management, particularly in patients with neurological symptoms or fluid balance concerns.”

Pediatric, obstetric, and oncology patients were excluded, limiting generalizability.

“Pediatric, obstetric, and oncology patients were excluded, limiting generalizability.”

POCT significantly reduces turnaround time by approximately 10 minutes on average compared with central laboratory testing.

“The results confirmed that POCT significantly reduces TAT by approximately 10 minutes on average, thus supporting its role in accelerating decision-making in time-sensitive clinical situations.”

• Also stated in: § Abstract · plain

  “TAT was significantly shorter for POCT (mean = 2.4 minutes) versus laboratory results (mean = 12.3 minutes), with a median delay of 10 minutes.”

POCT tends to slightly overestimate potassium compared with central laboratory measurement.

“Bland-Altman analysis revealed a mean positive bias of +0.12 mmol/L, indicating slight overestimation.”

• Also stated in: § Conclusions · plain

  “However, the observed tendency for POCT to slightly overestimate K⁺ and underestimate Na+ underscores the need for confirmatory lab testing when values are borderline or critical.”

POCT tends to underestimate sodium compared with central laboratory measurement.

“Na⁺ measurement showed a similarly strong correlation (r = 0.88; p < 0.001), though with a mean negative bias of −1.2 mmol/L, suggesting a tendency of POCT to underestimate Na⁺ levels.”

• Also stated in: § Conclusions · plain

  “However, the observed tendency for POCT to slightly overestimate K⁺ and underestimate Na+ underscores the need for confirmatory lab testing when values are borderline or critical.”

The observed discrepancies, although within accepted analytical thresholds, may still impact management decisions in borderline or clinically sensitive cases.

“Although the observed discrepancies were within generally accepted analytical thresholds, defined as ≤ 0.3 mmol/L for K⁺ and ≤ 3 mmol/L for Na⁺, they may still impact management decisions in borderline or clinically sensitive cases.”

A 1-2 mmol/L difference in sodium may lead to changes in therapeutic decisions in vulnerable or critically ill patients.

“Although a 1-2 mmol/L difference may appear minor, in vulnerable or critically ill patients, even slight underestimations may lead to changes in therapeutic decisions, particularly with regard to fluid restriction, diuretic use, or hypertonic saline administration.”

POCT using VBG analyzers provides strong analytical agreement with central laboratory testing for both K+ and Na+ in adult ED patients.

“The results of this audit demonstrate that POCT using VBG analyzers offers strong analytical agreement with central laboratory testing for both K⁺ and Na⁺ in adult patients presenting to the ED.”

• Also stated in: § Conclusions · boosted

  “This QI project demonstrated that POCT using VBG analyzers provides K⁺ and Na+ measurements with strong statistical agreement and clinically acceptable bias compared to central laboratory values in adult ED patients.”