You have a patient who is in acute renal failure. You have sent an ER-venous panel that shows a Cr of 4.6, a BUN of 95, and a K of 6.5. You discuss these findings with your nephrology colleagues who request you get the “more accurate” basic metabolic panel. But is it?
Answer: It depends on what you’re looking for.
For electrolytes: ER-VP
The reason: ER-VP uses a direct ion specific electrode that sends a current through an undilated sample of the patients whole blood. The voltage of that current varies depending on the concentration of the said ion. This voltage is compared to a standard. The result is a direct measure of ion concentration in the whole blood, which is ultimately more physiologically accurate.
The BMP, in comparison, uses similar ion specific electrodes, but measures the electrolyte content in volume of total plasma. It sounds similar, but in order to achieve this the whole blood sample must have the plasma separated out. Once achieved, the ISE measures the concentration of electrolytes in the water portion of the plasma (full of electrolytes) and the protein/lipid heavy portion of the plasma (less electrolytes). A weighted result follows and is reported.
In situations where a patient’s protein or lipid portion of their plasma is elevated or suppressed (think dehydration or malnutrition), this weighted average is subject to error. This lengthy process is also why it takes BMPs a long time to result.
For Creatinine: Stay tuned for the reason on Friday.
Ultimately if you have a patient who is in acute renal failure you should know that:
- In general, the two assays should read the same – in fact our lab goes to considerable effort to be sure they do within the precision of the methods – this is checked daily.
- Sending a BMP in addition to your venous panel may expedite an admission as upstairs teams often rely on them. If you do not order it upfront, BMP can be added onto any lab that was done in the yellow top (and panel being the most common).
- If there is any chance the patient has a Cr above 1.3 (which has to be about 60% of our patients), you should send a BMP to get a real Cr (more about this on Friday).
Former chief resident David Gutteridge, MD MPH who did an exhaustive search in 2016 as well as Dr. John McClaskey, M.D. of Mount Sinai Clinical Labs.
Here is an international consensus by the international federation of clinical chemistry (IFCC) regarding these two technologies. They point out the limitations of the direct method and suggest rigorous calibration standards. (http://www.ncbi.nlm.nih.gov/
This article analyzed the difference between the two techniques and estimated the discrepancy to occur in about 8% of samples. In each of these the direct measurement was thought to be the more accurate of the two. (http://www.ncbi.nlm.nih.gov/
Finally, here is a review article from way back in 1985 that argues that all labs should adopt the direct ISE because of its increased accuracy. (http://www.ncbi.nlm.nih.gov/