David Petts on laboratory pregnancy testing

David Petts on laboratory pregnancy testing
21 February 2019
David Petts, chairman of the IBMS History Committee and the Medical Sciences Historical Society, speaking in 2007 about his experiences of laboratory based pregnancy testing

When I started in bacteriology in 1965 we were doing a male toad test. Why were microbiologists doing hormone tests? The reason is simple: microbiology was responsible for the animal house. In those days we used animals for the isolation of some bacteria, and some bacterial toxins.  Toads are animals; therefore, it was a microbiologist's job to do pregnancy testing.  There was also a cynical view that it was an infectious disease with an infectious dose of one. We only did the tests when it was clinically important. I think diabetes kept coming up.    

Of course, there was the odd person that said, “Can you slip one in for us?” We only did a few, 6-8 a day and the lady needed to be at least two weeks overdue before it was considered suitable to test. The longer the better.  We needed at least 100ml of urine and the test took several hours. This was a lot faster than earlier tests but we would reckon between 6 and 24 hours were needed and, of course, the specimen had to come in the day before so we rarely did any urgent testing. There was no such thing a stat test. 

After numbering up, we would take the 100ml of urine and pour it into a 100ml measuring cylinder. That's how we knew we had 100ml. We would add to that 20ml of the kaolin suspension, about 5% I think it was. You could just get that 20ml into the 100 ml glass cylinder. And you would then leave them all and go and have coffee and half an hour later - generous coffee breaks - the kaolin would have settled. You would then decant the supernatant and recover the kaolin and put it into a heavy glass universal and centrifuge it for a few minutes to get the kaolin deposited firmly down.  Decant the supernatant and throw it away and re-suspend the kaolin in a little bit of weak hydrochloric acid.  The hydrochloric acid eluted the hormone from the kaolin.  Again it was centrifuged.  This time you kept the supernatant and neutralised it with NaOH against phenol red.  This you had to be very careful with, because if you didn't get it right you had to re-acidify the re-alkali. We had one lady who was very bad at this and I will tell you the consequences of that later. 

Once you'd got that you would go and get the toads.  They were male toads that were collected on the Norfolk marshes. We bought them from someone like Gerrards, who were people who supplied everything.  They were kept in a very small room in four boxes of peat, one box in use and three resting. They were fed on maggots. You'd go down there and get your toads and put them in a 2lb sweet jar with a little water in the bottle, number it all up, put the bottle of extract in front of the jar with the form, and then you'd go and fetch the pathologist. 

Things had moved on, or moved back - only pathologists were allowed to do this because they held the Home Office licence for dealing with animals and we were inspected regularly by the HO inspector - much more feared than CPA. The toad was injected into the back on the sac with the extract of urine and put back in the bottle in the water.  Now this is where if you hadn't neutralised it properly it kept swinging around things went wrong.  Because if you were neutralising HCl and with NaOH you ended up with a quite concentrated solution of sodium chloride (if your technique was bad) and if you inject that into a toad the sac acts as an osmotic membrane and the toad swells up. 

Anyway, having done that late morning, the toad was then left to just before tea (and you can see how minds work - lunch, tea, coffee) when the pathologist would appear and take a glass Pasteur pipette, which had been cut short and rounded off in the flame so it wasn't sharp, and catheterised this male toad. They would take some urine, place it on a slide and then look down the microscope and pronounce whether there were spermatozoa present.  If they were present, then the test was positive.  If there were no spermatozoa the test was not yet pronounced negative, the toad was re-incubated for 24 hours and re-sampled. 

The toad was then taken back down the stairs, into their cycle of boxes, and a report was sent out.  Most of the time, that was that. Occasionally, we'd get GPs phoning up demanding tests be done which weren't clinically necessary and asking why we needed so much urine. Once I had an Australian tell me where to put the telephone, and sideways, because I wouldn't give his result over the telephone.  His wife was going back to Australia and she wasn't going back if she was pregnant because he wasn't going to pay for her to have maternity care when he could get it for nothing in England. 

This went on from the late 1960s to the 70s when we started to use immunological tests. First Pregnosticon; that was a tanned red cell test and you got it settling down as a ring if it was positive, and a dot if it was negative. And if anyone banged the bench there was hell to pay.  And then Wellcome brought out their latex technique Prepurex.  This was latex labelled with antibody to hCG. A drop of urine was added to a slide, then a drop of anti-hCG serum, then latex coated with hCG.  No agglutination is a positive i.e. hCG in the urine has neutralised the anti-hCG.  Agglutination means negative i.e. anti-hCG not neutralised and it reacts with hCG coated latex.

Only in later versions did agglutination mean positive. All of these tests had a value of detection of about 200 units per ml, which was the cut-off that was used at that time. This had one further impact, because suddenly these tests were made available to pharmacists, and pharmacists started offering pregnancy-testing services over the counter.  And it was the first time that as state registered medical laboratory technicians we were then allowed to do a test that didn't have the specific request of a medical officer, because if pharmacists and nurses and everybody else can do it, so could we.  So many of my colleagues set up back-street pregnancy testing services and undercut the pharmacists.  Then the government said we can't have all this commercialisation, laboratories must do as many pregnancy tests as they possibly can, lots and lots of pregnancy tests.  We were doing up to 60-70 a day at its peak, but now it's dropped down because people just go to Boots and pee on something. 

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