The surfactant test is a simple way of estimating the maturity of a baby. It is seldom needed in the developed world, where the length of gestation can usually be obtained from a pelvic examination early in pregnancy, or by an ultrasound scan. Unfortunately, this does not apply in the developing world, where the surfactant test still has a useful place. It is not infallible, so don't rely on it alone[md]use it in conjunction with an estimate of gestation by dates, and an estimate of the baby's size. It is a test for the surfactant which his alveolar cells secrete, and which is necessary for the expansion of his lungs immediately after birth. If they don't expand, he develops the respiratory distress syndrome, so the test is a measure of the extent to which he is at risk from this.
The test normally becomes positive at 36 weeks, so it is a good sign that he is mature enough to induce. Obtaining amniotic fluid is easy and safe for him and his mother, and is no more ]]painful than an intramuscular injection. Rare complications include rupture of the membranes and injury to his head. Clements (see below) uses varying dilutions; we use only a 1:2 dilution.
Clements et al. The assessment of the respiratory distress syndrome by a rapid test for surfactant in amniotic fluid. The New England Journal of Medicine 1972;286:1077[nd]81. Fig. 19-1 THE SURFACTANT TEST normally becomes positive at 36 weeks, so it is a good sign that a baby is mature enough to induce.
THE SURFACTANT TEST INDICATIONS. (1) There should be a legitimate reason for induction, but not one which is so strong that you would induce the patient anyway, such as severe gestational hypertension (17.4). Indications include an elective Caesarean section with uncertain dates, suspected growth retardation (19.13), and diabetes (in which the surfactant test is somewhat unreliable, 17.3). (2) If you are able to use ultrasound to localize the placenta, and find that it is in the way of the needle, reassess the need for the test. In practice little harm results from going through it. If however she is Rhesus-negative, putting a needle through the placenta increases the risk of rhesus immunization. (3) She must have a mobile presenting part, showing that she has enough liquor to aspirate. If there is not enough liquor, he is probably mature enough anyway.
ASPIRATION. Take a sterile 10 ml syringe and a long 1 mm needle. If she is very obese, you may need an extra long one. Have a second syringe ready in case the first sample is blood- stained.
Ask her to empty her bladder, so that you don't aspirate her urine. There is no need for local anaesthesia. Prepare the skin over her lower abdomen, preferably with iodine. Lay her supine. The lowest part of the baby is usually his head: feel it, lift it up out of her pelvis as far as you can, and then hold it there with your left hand. This will allow liquor to swirl around it, and fill her lower segment.
While holding his head up, plunge the needle attached to the syringe into her uterus at right angles to the plane of her lower segment, as near to his head as is reasonable, remembering that you don't want to hit it. Remember also that the commonest complication is rupture of her membranes due to inserting the needle too low, too close to her cervix.
Alternatively, aspirate at the level of her umbilicus on the side of his limbs. You need to be able to feel his position clearly. There is usually a good pool of liquor there. Injuring him is very unusual.
Withdraw 5 to 10 ml of fluid. Record it as being clear, or blood-stained (indicating a traumatic tap), and the vernix in it as being absent, scanty, or plentiful. If you fail, try once more and then give up.
EQUIPMENT. You need: (1) 1 ml of clear liquor, uncontaminated by meconium or blood. Only the faintest blood- staining is acceptable[md]a pale ''ros[ac]e' colour at the most. If you cannot avoid blood contamination, centrifuge the liquor hard for 5 minutes and test the supernatant. (2) 95% alcohol. (3) Some completely clean glass test-tubes with an internal diameter 8[nd]14 mm. (4) ''Parafilm' to cover the tubes. If you don't have this, use new corks or rubber stoppers. If you don't have these either, a very carefully washed, and even more carefully rinsed, finger is probably better than a used cork or stopper.
METHOD. Take exactly 0.5 ml of liquor, 0.5 ml of saline, and 1.0 ml of alcohol (this mixes a 1 in 2 dilution of amniotic fluid with an equal volume of alcohol). Shake the mixture vigorously for exactly 15 seconds. Then don't move the tube. Wait 15 minutes before examining it in a good light against a dark background.
No foam, score 0.
An incomplete ring of bubbles peripherally round the meniscus, score 1.
A complete ring of bubbles round the meniscus, score 2.
As above, but foam just covering the whole meniscus, score 3.
Plentiful foam covering the whole meniscus thickly, score 4.
At a dilution of 1:2 as above a score of 1 or more means that his lungs are mature.
CAUTION ! (1) Avoid contamination with anything greasy. (2) Meconium produces a false positive result, so don't do the test if there is meconium in the fluid. (3) Don't shake the tubes a second time.
DIFFICULTIES [s7]WITH THE SURFACTANT TEST If you ASPIRATE NOTHING, have you been brave enough? Push the needle a little deeper and try again.
If you ASPIRATE BLOOD, it may be fetal or maternal blood. Check the fetal heart half-hourly for 4 hours. If it rises steadily, he is bleeding (rare). Section her immediately.
If her UTERUS BECOMES HARD, and there are other signs of placental abruption, she is bleeding behind her placenta (very rare). See Section 16.13.