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To Ultrasensetive PSA or standard PSA that is the question

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User

Posted 12 Apr 2019 at 13:26

This is a popular subject yet I don't think any of us know much about it. It would be good if a guest biomedical scientist would provide a write up.

I read a piece about how they do the test and it seems they machine is calibrated using a standard sample then it does its jiggerypokery. I'd be sure there will be a few manufacturers and they probably have different specifications as I once saw an advert for a machine that claimed to be more sensitive than others.

Also I'd feel the tolerance of accuracy would be no more than 10%, which on a 0.05 reading would be +/- 0.005. If it was +-0.02 it wouldn't be that useful even at 0.1.

I once read that 0.03 is a threshold above which errors are insignificant. Although they do say to get tested at the same place as there are differences in the calibration.

We are talking about 0.03 nano grammes per millilitre. 1 nano being a billionth of a gramme in a thousandth of a litre which is a millionth. Amazing!

Yes, If you prefer there are 2 Gypsy Petulengro's and you could see if there's a difference in their predictions. The daughter on the pier and the mother on the pleasure beach.

User

Posted 12 Apr 2019 at 14:43

Is PSA 100 percent consistent in our blood stream and would it be 100 percent consistent in the split blood sample Lyn's Dad provided ?

Thanks Chris

User

Posted 12 Apr 2019 at 15:25

I did speak to the actual biochemist in the pathology department at our ‘billion-pound’ super-hospital who carries out the PSA testing, and I asked her about super-sensitive assay and why they only give results to 0.1, i.e. <0.1, ‘undetectable’.

She was most helpful, and foolishly said:’If you need any more information, please come back to me’.

So I will be back in touch to find out more. She may live to regret it........

Will let you know make and model of the instrument, calibration schedules, etc., etc.

I don’t contribute here just out of benevolence, self-preservation is also involved....

Which Gypsy Petulengro is the most accurate, and when was she last calibrated?

Cheers, John.

User

Posted 12 Apr 2019 at 21:28

Originally Posted by: Online Community Member

Is PSA 100 percent consistent in our blood stream and would it be 100 percent consistent in the split blood sample Lyn's Dad provided ?

Thanks Chris

My professor of pathology friend told me that it would depend which bit of the sample you were looking at on the slide.

"Life can only be understood backwards; but it must be lived forwards." Soren Kierkegaard

User

Posted 12 Apr 2019 at 21:37

Something something blind man at the dartboard

User

Posted 12 Apr 2019 at 22:36

In reply to Chris about whether the blood in one arm will have the same level of psa as in the other. I'd say yes because blood is pumped around the body many times and likely to equalise. Although there are probably better reasons. If a sample was split and put into the same analysis it would also be the same as the machine pumps the blood many times through a sensor, or so it appears. To find the bit in a million probably takes a few swills.

To John as gypsy forecasts are pretty random I'd go to the daughter but it's a matter of taste and PCness. Looking forward to the self preserving non benevolence.

Edited by member 12 Apr 2019 at 22:37 | Reason: Not specified

User

Posted 12 Apr 2019 at 22:40

Originally Posted by: Online Community Member

If a sample was split and put into the same analysis it would also be the same as the machine pumps the blood many times through a sensor, or so it appears. To find the bit in a million probably takes a few swills.

In Dad's case, one sample was split and tested twice = two different results.

In John's case, sample given at 8.30am and another at 9am (in different places but both tested at the same lab) = two different results

"Life can only be understood backwards; but it must be lived forwards." Soren Kierkegaard

User

Posted 12 Apr 2019 at 22:43

I have Tarot cards and the I Ching if that helps....

User

Posted 15 Apr 2019 at 17:07

Maybe I can shed some light on what can be expected from an analytical method and a PSA measurement.

There are two parameters that characterise any method used to measure the amount of any chemical in blood be it a natural component e.g. PSA or an administered drug e.g. aspirin. When the analytical method is being validated the biochemist determines the accuracy and the precision of the method. The accuracy is a measure of how close the measured value in a blood sample is to the true value (which can never be known - only estimated). The precision is the scatter of values obtained when exactly the same sample is measured a number of times. It is highly likely that if the same blood sample is analysed 10 times then 10 different values will be obtained. The aim of the biochemist developing the method is to minimise these variables.

Usually validated analytical methods will will be accurate to within e.g. 5% of the true value. Likewise the measured value will be expected to fall within a range expressed as plus or minus e.g. 5%.

So when we get a PSA value reported as 10 ng/ml the true value will lie somewhere between 9.0 ng/ml and 11 ng/ml.

I would encourage all not to get hung up on small differences in PSA values.

My last three values were 2.7 ng/ml (april 2018) 2.5 (august 2018) 2.8 ng/ml (january 2019). As far as I am concerned these values are essentially the same and the differences are a reflection of the random errors inevitable in any analytical measurement.

User

Posted 15 Apr 2019 at 18:54

There must be millions of stored blood samples in the US. In the case below the manufacturer only supplied calibration down to 0.1 whether that was the limit in 2004 or if it was a cheaper machine I don't know. The machine does the calculations.

Here's an extract from a Lab Procedure dated 2004 along with the link to the full manual:

The instrument automatically calculates all results. After testing is completed, results are printed and review by the technologist. Samples with results > 150 ng/mL are diluted off-line and repeated; results are corrected for the dilution factor. Samples with results < 0.1 ng/mL are repeated to confirm.

Results are reported to the nearest tenth (0.1). The lowest reportable PSA result is 0.1 ng/mL. The assay does not have a maximum reportable limit since off-line dilutions can be made to bring the concentration within the working range of the assay. Estimates of imprecision can be generated from long-term quality control pool results.

The upper reportable value is virtually unlimited. The upper limit for undiluted specimens is determined by the calibration material supplied by the manufacturer. Values exceeding this upper limit are repeated on dilution until values, prior to correction for dilution, fall between approximately 5.0–150.0 ng/mL B. The lowest reportable value is approximately 0.1 ng/mL. The lower limit of this assay’s default dilution is determined by the calibration material supplied by the manufacturer. Values below this lower limit are repeated to confirm the result.

The residual serum is stored at ≤ –70o C for 6 months after analysis, then it is returned to the NHANES Repository in Rockville, MD for long-term storage.

https://www.cdc.gov/nchs/data/nhanes/nhanes_03_04/l11psa_c_met_total_psa.pdf

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