CAP Proficiency testing on the i-STAT/pCO2 negative bias

We have had a persistent negative bias for pCO2 on the CG4+ blood gas cartridge for the past year.  I have reached out to Abbott for their recommendations and haven't heard anything back.  Has anyone else experienced this and what have been your corrective actions?  I am concerned because I have also had no response from Abbott regarding this study that was done a very long time ago.

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Am J Clin Pathol. 2000 Jul;114(1):128-38.

The rise and fall of i-STAT point-of-care blood gas testing in an acute care hospital.

Ng VL1Kraemer RHogan CEckman DSiobal M.

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In response to a $350,000 laboratory budget cut and closure of an intensive care unit-based laboratory and a desire to maintain turnaround times of 10 minutes or less, a multidisciplinary group developed and implemented point-of-care (POC) testing. Only blood gases (pH, PO2, and PCO2) and ionized calcium values were deemed essential stat tests. Three commercially available POC blood gas devices were evaluated; all yielded results comparable to in-house reference methods. The 1 device with a US Food and Drug Administration-approved method for ionized calcium testing and with an existing interface for laboratory information systems was selected. Fiscal analysis predicted annual savings of approximately $225,000. POC blood gas analysis was implemented in April 1996 coincident with closure of the intensive care unit-based laboratory. Clinical laboratories and POC blood gas test volumes remained constant through August 1998; in contrast, the number of ionized calcium tests decreased dramatically after April 1996. In August 1998, clinically significant (i.e., artificial ventilation parameters would have been altered based on test results) discrepant PCO2 values were observed sporadically and noted only with patient specimens, not with commercial controls or electronic simulators. Because investigation failed to identify the cause, use of the POC device was discontinued in September 1998.

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We are seen a negative bias too for pO2 and pCO2. We will be contacting CAP as well as Abbott regarding tis issue.


Thanks for reporting it.

Hi Vivian, it's frustrating Abbott isn't responding.  We've retested the spare set of samples after submission to see what an med tech could get and our numbers were again on the low side of the mean.  We have three experienced perfusionists that perform this test and have it rotated among.  This is the second time we have an unacceptable grade on the pCO2 (last time was 2017) and we were one point out!

What internal RCA have you done for this?  We do have a process where we work through eliminating clerical error, sample handing error (possible), systemic error etc.

Hi Karen, I just called Abbott and opened a case for them to investigate our issue. We are using the EG+7 cartridges and the pCO2 is on the low side on the original and repeated samples. CAP also suggested to send them the documentation to show the bias in your institution. Right now the operator enters the data directly into the CAP answer sheet. We are going to have a pWEB print out to make sure there is no transcription error. We are going to monitor closely the technique. Document the time the samples are at RT, the time they were tested and make sure there are no bubbles.



We failed pCO2 on the low side for the first time on one analyte this past survey. All our pCO2 results the past year have the negative bias so I know pathology will want more info. My question for the group is what cartridge type is used. We use the CG4 for all CAP samples as it covers the range of analytes. I do see that CG8 cartridges run slightly higher on QC material. If the majority of people run CG8 then those of us who do CG4 may always be lower. Thoughts? I have a call into Abbott but have not heard back yet. ivy

Our hospital uses G3, CG4, EG7, CG8, Chem8, and more cartridges for the I-Stat. For the Critical Care Blood gas Istat proficiency test I run the samples as followed and have never had a problem.

first set of vials are run on CG4 to obtain all blood gases and Lactate

immediately opening a vial we follow it up by running a Chem8 cartridge to get the remainder of the analytes.

With the second set of vials I run EG7 cartridges to get my HCT values. I also use these values if I had any questions on the validity of the first set of blood gases.

Biggest thing is to pop the tops and load immediately, any excess time really messes up your pCO2 an O2 results.


We also failed this last proficiency survey but for the pO2, and we are also frustrated because we've noted the negative bias for this analyte on the iSTAT along with pCO2.  We use the G3 cartridges and the sensitivity of the sample leaves no room for any handling error but we make sure to rotate the surveys amongst our staff, even the ones who don't test as often.  I will follow suit and contact Abbott and CAP about the issue because even with the QC, the numbers don't look very good. 

We use the CG4+.  The POCC tested the spare set (after submission of course) and got the same results, pCO2 on the low end and one point for being out.  We are ordering a set of API proficiency samples to see how we do there. 


Your proficiency testing process seems to violate the rule against running samples in duplicate.  Have you been inspected since you started this?

I am not the one running the samples so they are never duplicated nor are results changed. CG4 cartridge does not have the other analytes needed to be tested so other parties will have to use the other vials provided to run those analytes. All analytes are tested with the two sets of vials provided. When EG7 are used they are only used to give the HCT result. There is no way to get a Lactate result and a HCT result without having to run 2 different cartridges.  

We were told that running the same analyte twice (even by different people on different cartridges) is a violation if it’s done before the submission date.  The EG7 cartridge has analytes which are duplicated on CG4 and CHEM8.

Hmm that's a good question, I will have to research this further. I am unable to use Chem8 here for my HCT and HGB bc. that analyte is turned off in our ED. Our ED is the only department that uses Chem8, and they have chosen to only run CBC's for those results. The only cartridges that reports HCT are CG8's and EG7's and neither of those report lactate.

Terry is correct Alayna you can't run PT twice for the same analytes, my suggestion would be to only use CG4 and CHEM8 - you should have an H/H on your CHEM8.

Another point, if your main lab uses a different method and you consider them the "primary" you don't need to do PT for iSTAT. We consider POC to be the primary for blood gas, but the lab is primary for chemistry and heme, so we only do blood gases now on the PT.

Third point - glass vials are affected by altitude. I send CAP a long letter about how their decision to remove the peer group barometric pressure for PO2 was going to destroy Colorado - and I was right. We have a constant negative bias if we don't correct the PO2 for barometric pressure. I just note it every survey - CAP and Abbott are aware.

For PCO2 incidentally, ours were all good in Colorado this round. So it's interesting that everyone else had a low bias. In any case, glass ampules do not perform the same as real patient samples. I always fall back on that, because I have the luxury of a lab with a different method that I can compare whole blood patient samples to. If you also have that, I would do a patient comparison. If those look good, you know the issue is not the iSTAT, it is the matrix of the CAP samples.

Here in Delaware…altitude 128’…we’ve also noticed a negative bias for pCO2 and pO2, but have had no PT failures.  For pCO2 the “B” survey from 2017 looked good, but 2017 “C” and 2018 “A” had an average bias of -10%.  For pO2, 12 of the last 15 results have been below the mean.


Our biggest problem is with Hgb/Hct.  We run Hgb/Hct on CHEM8 cartridges, and results show a very negative bias.  Abbott states that the sensors are equivalent across all cartridge types.  Our experience shows that CHEM8 gives lower Hgb/Hct results than EG7 or E3.  Even the value assignment sheets reflect this difference.

Hi Terry, yes we have seen the same with the H&H on the Chem 8+ with a negative bias on the CAP proficiency.  Is anyone using API or Wisconsin proficiency kits and seeing the same?

We just switched to API for ISTAT this year. For the 2 events we have had so far, both had 2 of the 5 samples slightly on the negative side. The other samples were either right on the mean or slightly on the positive side. We use Chem8 cartridges for the H/H.

Just thought about the PO2 issue; our first event was on either side of the mean (I think it was 2 neg and 3 pos if I remember correctly). The second event were mostly on the negative side but not by much. How the sample is applied to the cartridge impacts the results also. Those that do not follow the steady application of the sample have different outcomes from those that follow their training.

Has anyone that has contact CAP or Abbott gotten a response?

We use G3 cartridges and have a low bias as well, I don't question the sampling/storage techniques as I observed every sample performed. We don't report out pO2 for patient testing but do use SO2 which uses the PO2 for the calculation.

We are ordering a set of API proficiency samples to see where we fall in that peer group.  We opened a ticket with Abbott to investigate and they had us send a box of the CG4+ to them.

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