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So you think you understand pregnancy tests?

Less than 40 years ago pregnancy was typically diagnosed by history and examination alone. While clinical skills always remain useful, there have been major advances in pregnancy testing that have been both clinically and medicolegally important. Like all diagnostic testing, pregnancy tests are not infallible, and it is very helpful for clinicians to understand their strengths and weaknesses.

Human chorionic gonadotropin (hCG) is redundantly called ‘human’ but appropriately named as the gonadotrophic hormone produced by the invading chorionic cells during implantation of the fertilised ovum. Like the pituitary gonadotropins, FSH and LH, hCG is made up of an identical alpha subunit but different beta subunits. Therefore old designation of ‘beta’ hCG, while just as redundant as ‘human’, is still commonly used, although in laboratory speak it could refer to the intact beta subunit rather than the beta subunit fragments that can appear after degradation, particularly in the urine.

Like most circulating proteins, hCG may have carbohydrate residues attached to it and hyperglycosylated hCG is a form of hCG commonly produced by gestational tumours (choriocarcinoma or hydatidiform mole), but is also produced in the very early stages of implantation. Not all laboratories run hCG assays that can detect the hyperglycosylated form of hCG and may poorly identify the presence of gestational tumours.

Pregnancy tests detect the more common forms of hCG, and while they are also internationally standardised to give similar values in blood or urine, they do slightly vary in their results. This is important when trying to compare two results – they should always be obtained from the same assay. The variation between assays may also be why typical ranges of hCG for the later stages of pregnancy vary from laboratory to laboratory.

What is a positive pregnancy test?

Implantation of a fertilised ovum typically occurs around seven days after ovulation i.e. on Day 21.

It makes sense therefore that hCG levels typically start to rise well before the missed period. In successful implantation, hCG rises very rapidly before the time of the missed period.

Yet why do we generally recommend testing from Day 28 (the day of the missed period) and onwards? Just because implantation has occurred, there is no guarantee that pregnancy will progress. A positive hCG at the time of the missed period is more a reliable indication that early implantation has been successful and testing at this time causes less maternal and clinical concern with regard to pregnancy loss.

The level of serum hCG that is called a positive pregnancy test is usually 15IU/L. This has decreased, from 50 and 25IU/L, over the years as assays have improved their sensitivity to low levels of hCG. Urine assays, particularly over the counter tests, have poorer sensitivity and still may have positive thresholds of 20IU/L.

The level that is currently called equivocal for blood tests is 5-14IU/L and therefore a negative pregnancy test is a level below 5IU/L.

Laboratories often quote a reference interval of <3IU/L, so doesn’t that mean that a level of 4IU/L must be positive? The answer is, the same as the paragraph above, that a level of 4IU/L may indicate (with some certainty) that implantation has occurred, but that level is a poor indicator of a successful pregnancy, especially at the time of the missed period.

IVF physicians often look for levels above 3IU/L, however

With the ever increasing maternal age at the time of pregnancy (esp. in IVF), it has become increasingly important to recognise that non-pregnant perimenopausal women can have hCG levels in the equivocal range, causing clinical confusion. The cause is related to the high levels of FSH and LH in perimenopause and is thought to be the perimenopausal production of ‘pituitary hCG’. Laboratories often quote reference limits of 7IU/L or greater in perimenopausal women.

What is a reassuring hCG level?

In a successful pregnancy, hCG levels double every two or three days, but this only occurs in early pregnancy. By 10-12 weeks gestation, hCG levels have plateaued and then naturally start to fall.

Only use hCG rate of rise while hCG levels are below 1000IU/L.

This is also convenient because from that point on, the gestational sac is typically visible on ultrasound and that is a far better indication of pregnancy viability.

In summary, the absolute level of hCG should not be used to judge viability.

Levels of hCG far greater than expected for that stage of pregnancy may be due to wrong dates, multiple pregnancies or gestational malignancy, making ultrasound important.

A falling hCG level may be normal after the first trimester, but in early pregnancy it can indicate poor prognosis. As hCG should be doubling every two or three days (when hCG is below 1000IU/L), it makes sense that a level that isn’t rising is worrying.

This is of particular importance in possible ectopic pregnancy.

When a pregnancy has been lost, and hCG is no longer produced, the disappearance half-life of hCG is 2-3 days. In early pregnancy, if hCG is not falling as quickly as expected, it is possible that the pregnancy has been lost but there are retained products.

Conclusion

No diagnostic test is infallible, and while hCG tests are among the best of immunoassays, false positives and false negatives occur.

More recently, patients taking high dose biotin (e.g. in Multiple Sclerosis), can cause false negative hCG levels.

While hCG testing has advanced in its sensitivity, specificity and we have more confidence in interpreting levels, in far less than 1% of cases, hCG levels may be unreliable.

It will always be important that clinicians question any discordance between clinical history and examination and hCG results and either conduct confirmatory testing or contact the chemical pathologist employed by that laboratory who can help with all these questions.

General Practice Pathology is a regular column each authored by an Australian expert pathologist on a topic of particular relevance and interest to practising GPs. The authors provide this editorial free of charge as part of an educational initiative developed and coordinated by Sonic Pathology.