I have talked a lot about how delayed processing times can impact the reliability of the serum potassium value, but what about other values, like phosphorus? If you don’t want to read on – the answer is yes! Phosphorus is sensitive to time delays in processing.
In today’s post I am taking a closer look at how delays in processing time can impact other lab values, including phosphorus. Today’s journal club article is: Omar, Julia, et al. “Effects of Time Delay in Processing Common Clinical Biochemical Parameters in an Accredited Laboratory.” IIUM Medical Journal Malaysia 21.4 (2022).
Why the authors did this study?
We rely a lot on lab values. We need these lab values to correct, so that we can give our patients the right advice. But as these authors point out, lab errors can occur. 48-68.2% of errors occur in the collection phase. 18.5-47% of errors occur after analysis. But what about errors DURING processing? And how does the time to processing impact some of our most common lab values?
What did they do?
The authors collected blood samples from 40 hospitalized patients. Each patient provided 4* 2mL of blood.
The samples were analyzed at 1, 2, 6 and 24 hours after the blood draw. The 1 hour sample allowed to clot at room temperature for 30 minutes and then centrifuged. All other samples were kept at room temperature for the pre-determined time and then centrifuged.
They analyzed the blood for:
- Sodium
- Potassium
- Magnesium
- Phosphate
- Calcium
- Urea
- Creatinine
- Uric Acid
- Total protein
- Albumin
- ALT
- AST
- ALP
- Total Bilirubin
- LDH
What did they find?
The main outcome were that lab values changed over the 24 hour period. However urea, creatinine, uric acid, and the liver function test panel (except LDH) were the most stable. All other values had statistically significant changes at each time point.
Below is a summary of the articles Table 1 and Table 2.
| Lab Value | % Change @ 2hrs | % Change @ 6hrs | % Change @ 24hrs | Acceptable Delay |
| Sodium | 0% | <1% | 1.69% | <6 hours |
| Potassium | 0% | 2.56% | 5.13% | <2 hours |
| Magnesium | 2.15% | 4.30% | 6.45% | <2 hours |
| Phosphate | <1% | 2% | 22.96% | <6 hours |
| Calcium | <1% | 1.93% | 1.93% | <2 hours |
| Urea | 0% | <1% | 2.91% | <24 hours |
| Creatinine | <1% | <1% | <1% | <24 hours |
| Uric Acid | <1% | <1% | 2.41% | <24 hours |
| Total Protein | <1% | 1.35% | 1.95% | < 6 hours |
| Albumin | 1.08% | 1.35% | 1.35% | <24 hours |
| ALT | <1% | <1% | <1% | <24 hours |
| AST | 1.22% | 2.43% | 3.65% | <24 hours |
| ALP | 1.82% | 4.59% | 6.53% | <24 hours |
| Bilirubin | <1% | 1.94% | 0% | <24 hours |
| LDH | 6.42% | 13.61% | 18.95% | <2 hours |
The authors conclusion
At 2 hours potassium, magnesium, calcium and LDH were clinically impacted. At 6 hours total protein, phosphate and sodium were impacted. The others tests were impacted after 24 hours.
Limitations: Differences in test tubes, room temperature and laboratory machine and maintenance may all be factors that could impact the reproducibility of these results.
My take aways
This article is not the first to report that delays in centrifuging blood samples can lead to changes in serum phosphate levels. In fact, this article reports that elevations in potassium and phosphorus can actually serve as markers of delayed centrifugation (as opposed to markers of someone’s serum level).
Many CKD patients have higher phosphorus levels. This could make a fake hyperphosphatemia harder to spot. So if you notice, that a value is outside of your patient’s normal trend, look for a reporting delay. If there was a delay, it is possible that you are looking at a lab error as opposed to a true result.
