sphingomyelin ratio and the rapid surfactant test in relation to fetal pulmonary maturity

A prospective evaluation of the lecithin/sphingomyelin ratio and the rapid surfactant test in relation to fetal pulmonary maturity RICHARD M.

L.

NEIL MARY J.

C.

KENISTON,

PERNOLL,

M.D.,

R.

M.

ROBERT

Portland,

BUIST,

LYON,

B.S. F.A.C.O.G. M.D.,

CH.B.,

M.R.C.P.E.,

SWANSON,

PH.D.

Oregon

Fetal lung maturity was correlated obtained over a 2% year interval. metric and densitometric methods the planimetric L/S ratio (P L/S) positively in a highly significant in predicting fetal lung maturity most reliable test for fetal lung

to the results of amniotic fluid analysis in 578 samples Lecithin/sphingomyelin (L/S) ratios obtained by planiwere compared to the rapid surfactant test (RST). Both and the densitometric L/S ratio (D L/S) correlated

fashion w&h the RST. Both RST and D L/S were reliable (99 and 100 per cent, respectively). The RST was the immaturity (69 per cent). Therefore, this series indicates

that the RST could be used as a primary method tional indicator of fetal maturity when the RST nated with blood and/or meconium.

with the D L/S was intermediate,

S I N c E T H E lecithin/sphingomyelin (L/S) ratio in amniotic fluid was introduced by Gluck and co-workers1 as a test for fetal maturity, the technique, its interpretation, and significance have generated considerable controversy. Most authors have agreed that the L/S ratio is a useful test for fetal maturity.“-I5 Many, however, have reported discrepancies between the results of the test and the clinical status of the infant, particularly when the

This work r.vas supported of Health Trainina Grant &ant from The N>tional March of Dimes.

by National HD-00165 Foundation-

Received

April

Accepted Reprint Division Oregon

for publication July July

being useful as an addior the sample contami-

L/S ratio is in the intermediate range ( 1.5 : 1.0 to 2.0: 1.0) .3-1s Nakamura and Roux and their colleague+ IT argued that the test is not a reliable index of fetal lung immaturity, and suggested that the L/S ratio may reflect nonpulmonary sources of phospholipids and gestational age rather than just pulmonary maturity. This view has received support in animal work by Condorelli and co-workers.ls Furthermore, there have been reports of infants with L/S ratios greater than 2.0: 1.0 developing respiratory distress syndrome (RDS) or hyaline membrane disease (HMD) or dying from respiratory complications.11-15 We, therefore, watched the developing controversy with interest, while we experimented with the technique, applying the modification of Borer and When Clements and a.ssociateP’ associates.‘g reported their results with the rapid surfactant test (RST) , which was both less tedious and potentially, at least, as accurate as the standard L/S ratio, we designed a study to compare the two methods prospectively in relation to fetal pulmonary maturity.

From the Divisions of Perinatal Medicine and Medical Genetics and the Defiartments of Pediatrics and Clinical Pathology, Uniuersity of Oregon Medical School.

Revised

D.C.N.

R.N.

Institutes and by a .

19, 1974.

5, 1974. 26, 1974.

requests: M. L. Pernoll, M.D., Head, of Perinatal Medicine, University of Medical School, Portland, Oregon 97201. 324

Volume Number

121 3

Maferials

Evaluation

and

methods

The patients involved in this study were cared for by the University of Oregon Medical School Perinatal Services. Most of the pregnancies were at high risk. The most frequent indications for amniocentesis were : repeat cesarean section, fetopelvic disproportion, pre-eclampsia, isoimmunization, diabetes mellitus, fetal postmaturity, premature rupture of membranes, and potential fetal jeopardy (from a number of causes). Amniotic fluid was obtained in each case by transabdominal amniocentesis. The fresh sample was centrifuged at 2,000 r.p.m. (600 x g) for 15 minutes and the supernate of the sample was used for all tests. If the sample could not be analyzed immediately, it was frozen. Samples which were grossly bloody and/or meconium stained were not excluded from the study. Contaminated samples comprised approximately 15 per cent of the study material. Methods were altered during the course of this study (Table I) in an attempt to delineate the better method. However, within each of the five study periods, the personnel, equipment, and techniques were constant. Planimetric lecithin/sphingomyelin ratio (P were determined by thin-layer chromaL/S ) tography (TLC) according to the method of Borer and Gluck and their colleagues.lp I9 The relative size of the chromatographically obtained phospholipid spots was determined by measuring the transverse and longitudinal diameters of the spots and computing the product of these two values. Each P L/S determination was done in duplicate. Densitometric lecithin/sphingomyelin ratios (D L/S) were determined by a modification of Borer and Gluck’s method. Prior to phospholipid extraction, two dilutions were made, 1:l and 1:3 with normal saline, giving three aliquots from each sample. The phospholipids were extracted as for the planimetric method except that acetone precipitation was n.ot employed. This modification was suggested by Sarkozi and associates.2’ The reconstituted phospholipids were spotted on seven-channel TLC plates (Helena Laboratories) . Chromatography was the same as the planimetric method. Phospholipid spots were detected by dipping the entire TLC plate in 10 per cent H,SO,. A densitometer (Clifford Instruments model M345) was used to scan the chromatograph and record the concentrations of lecithin and sphingomyelin. The ratio of lecithin to sphingomyelin was calculated. The average of the three values obtained for each

of

L/S

Table I. Analytical

Method A. B. C. D. E.

ratio

methods

of analvsis

P L/S only P L/S + RST RST only D L/S only D L/S f RST

and

surfactant

No. of amniotic fluids analvted 1972 1972 1972 1972 1973

131 55 18 38 336

Total

578

Table II. Method maturity

Intermediate

Immature

of interpretation

of fetal

lung

tests

Predicted fetal status Mature

325

employed

Dates June 1971-June June 1972-Sept. Sept. 1972-Oct. Oct. 1972-Nov. Nov. 1972-Dec.

test

L/S

ratio

2 2.00

1.50-1.99

<

1.50

RST Complete ring of bubbles persisting 15 min. at the 1: 1 and 1: 2 dilutions Complete ring of bubbles persisting 15 min. at the 1: 1 dilution only Incomplete ring of bubbles a; both dilutions

specimen was recorded as the D L/S. The method of interpretation of the L/S ratio was the same for both methods (Table II). The rapid surfactant test (RST) was that of Clements and associates,Z0 but only two dilutions of amniotic fluid with 95 per cent ethanol were used: 1:l and 1:2 (amniotic fluid:ethanol). The tubes were capped with Parafilm and shaken vigorously for 15 seconds on a Vortex mixer. The criteria of Clements and associates?” regarding the significance and types of bubble patterns were employed (Table II). The gestational age of the infant was determined by the pediatrician attending at birth, employing the criteria of Lubchenco.22 The gestational age of the fetus at the time of amniocentesis was calculated from this value by subtraction of the intervening time span. Table III summarizes the criteria used for the respiratory distress syndrome (RDS) .23 Results

Estimation of fetal age by the L/S ratios and the RST. The relationship between the P L/S and the length of gestation lationship between the is illustrated in Fig. 2. termining the L/S ratio viously published result

is shown in Fig. 1. The reD L/S and the fetal age These two methods of deare compared with a preof the reflectance densito-

326

Keniston

February 1, 1975 Am. J. Obstet. Gynecol.

et al.

Table III. Criteria

Table V. The relationship

for RDS

RDS classification Some had complications as listed under mild RDS, but these lasted less than 6 hr. Mechanical obstruction to respiration, apnea, and/or asphyxia present in some cases Some infants did require oxygen

None

Mild

Grunting, tachypnea, sternal and intercostal retractions, nasal flaring persisting longer than G hr. Arterial partial pressure Paoz > 50 mm. Hg jn 50% Oz Metabolic monitoring and support, plus oxygen necessary to support life

Moderate

Severe

Pace, < 60 mm. Hg As above, but rrquiring continuous positive airway pressure (CPAP) to maintain Pao, at or aljove 50 mm. Hg in 50 per cent 0, .Is

above, but also requiring manually assisted ventilation or respirator support to maintain Paos < 60 mm. Hg 1)evelopment of classical radiographic cphanges in the lung fields If the infant died, he or she was considered to have had severe RDS if the lungs were immature, or HMD was present at autops-v

Table IV. A comparison determining

between the clinical and the P L/S, the D L/S,

status of the neonate and the RST”

the L/S

25 26 27 28 29 30 31 32 ::3

0.52 0.55 0.702 0.82 0.62 0.82 1.0 1.0 1.1

+ 0.18 0.18 5 0.11 -I 0.20 + 0.20 + 0.48 f. 0.51 + 0.38

::4

1.5

20.4

::5 36 37 38 39 40 41 42 43

2.1 2.3 2.9 3.5 3.8

? 0.2 + 0.3 to.6 ?: 0.7 i: 1.2

5.2

i: 1.4

8.1

22.0

*Cluck’s results results, using both

of methods

for

ratio*

are from methods,

1.35 1.42 1.94 1.68 1.67 1.54 2.16 (mode 2.47 (mode= 1.81 2.86 2.67 3.82 4.12 4.62 3.11 5.68 5.45

5 0.41

f k + = f

normal involve

0.94 0.23 1.52 1.50) 1.81 1.75) + 0.99 + 1.22 + 1.35 t 2.42 + 1.91 + 3.61 + 1.47 f 3.20 2 1.67

0.85 0.89 0.83 0.89 0.93 0.60 0.89 0.86

?r 0.08

t f + f f f

0.16 0.18 0.41 0.16 0.24 0.2 7

1.19 + 0.51 0.99 1.24 1.29 1.77 1.81 2.15 2.25 2.08 2.01

+ 0.24 + 0.40 ? 0.50 _+ 0.83 + 0.53 ? 0.69 + 0.51 + 0.32 2 1.00

pregnancies only; our high-risk pregnancies.

Predicted Mature RDS

No.

P L/S (67 cases): No RDS 50 Yes 3 D L/S No Yes

(143

RST (151 No Yes

1

fetal

status

Intermediate 9%

1 No.

1

Immature 95

1 No.

/

7%

94.3 5.7$

6 3

66.7 33.3

3 2

60.0t 40.0

cases): 81 0

100 O.O$

32 5

86.5 13.5

17 8

68.0? 32.0

cases): 109 1

99.1 0.9$

23 2

92.0 8.0

5 11

31.2t 68.8

*Only cases delivered within 72 hours of amniocentesis are included. The numbers in each category as well as the percentage (relative reliability) for that category are illustrated. The areas of “false negatives” (t) (i.e., where the test indicated RDS would occur and it did not) and “false positives” ($) (i.e., where the test indicated RDS would not occur and it did) are indicated.

metric method of Gluck and Kulovich2 in Table IV. Gluck’s results are from normal pregnancies. Comparison of the P L/S and the D L/S reveals that both the mean value of the L/S ratio at a given week and the variation about the mean are much less for the D L/S. In the interval from 35 to 40 weeks, the logarithm of both L/S ratios is linearly correlated to the fetal age: D L/S, r = 0.65, p < 0.001; P L/S, r = 0.52, p < 0.001. The relationship between the results of the RST and the age of the fetus is shown in Fig. 3. Between 37 and 40 weeks of gestation, the percentage of fetuses which appear to be mature by the RST increases from 16 to 76 per cent. In the 33 to 41 week interval, there is a highly significant linear correlation between the RST and the fetal age (r = 0.62, p < 0.001).

The pulmonary status as predicted by the L/S ratios and the RST. Table V compares the pulmonary status of all infants delivered within 72 hours of amniocentesis to the analysis performed. The P L/S gave nearly 6 per cent false positives (that is, fetal maturity was indicated by the test when, in fact, the fetus developed RDS), while the D L/S gave no false positives. Two of the infants whose maturity was falsely predicted by the P L/S developed severe RDS and died. The third infant developed only mild RDS. Both methods of determining the L/S ratio gave false negatives (that is, fetal immaturity was indi-

Volume Number

121 3

Evaluation

Planimetric

L/S

Ratio

‘Densltometric

of

L/S r

L/S

ratio

and

surfactant

Ratio

test

.

..

.s.

T

l

3 00

.

. l

.

.

l

.*

t

.

l

.

.

_

.

: . l .a

..-

i

:

327

.

l

l.

.*

. lmmoiure

Weeks

Fig. 1. The gestation.

The

P

L/S results

Gestation

ratio as a function are

of I86

duplicate

Weeks

of the weeks analyses.

of

cated when, in fact, the infant did not develop RDS). The D L/S gave the higher percentage of false negatives. The degree of linear correlation between the P L/S and the pulmonary status of the infant was significant (r = 0.25, p < 0.05), whereas the degree of correlation between the D L/S and the pulmonary status of the infant was highly significant (r = 0.31, p < 0.005). Only one infant in the 110 judged mature by the RST developed mild RDS. The percentage of false negatives in the immature category is lower than for either of the L/S ratio methods. The RST correlates linearly with the clinical status of the neonate in a highly significant fashion (r = 0.39, p < 0.001). The correlation between L/S ratios and the RST. Table VI compares the results of the P L/S-RST combination in 55 cases and the D L/S-RST combination in 336 cases. Of those judged mature by the RST, 96 per cent were also judged mature by the P L/S. The over-all concordance between the RST and the P L/S was 80 per cent. The P L/S and the RST correlate linearly in a highly signifi0.65, p < 0.001). Of those cant fashion (r =

Gestotlon

Fig. 2. The D L/S ratio as a function gestation.

The

results

RAPID

El

of the weeks analyses.

are of 374 triplicate

SURFACTANT

of

TEST

Immature

lnlermediole

Number of samples 17 15 15 22

onoiyred: 25 31

Mature

43

53

55

83

23

16

37

38

39

40

41

42-43

Percen of Cases

25-31

32

33

34

35

Weeks

Fig. 3. The RST as a function fraction week.

of

cases

in

each

RST

36

Gestation

of gestational category

age. The

is indicated

by

328

Keniston

Febxuary 1, l!Ii: Am. J. Obstet. Gynccol.

et al.

VI. Correlation

Table Result RST

between

the RST

and both

P L/S

of Result

1

No.

of cases

%

methods

for L/S

Result of RST

ratios D I./S

Result

1 No. of cases

1

75

Mature Mature Mature

Mature Intermediate Immature

24 1 0

96.0 4.0 0.0

Mature Mature Mature

Mature Intermediate Mature

79 29 17

63.2 23.2 13.6

Intermediate Intermediate Intermediate

Mature Intermediate Immature

3 3 0

50.0 50.0 0.0

Intermediate Intermediate Intermediate

Mature Intermediate Immature

18 32 34

21.4 38.1 40.5

Immature Immature Immature

Mature Intermediate Immature

3 4 17

12.5 16.7 70.8

Immature Immature Immature

Mature Intermediate Immature

0 10 117

0.0 7.9 92.1

Total

55

judged mature by RST, 63 per cent were also judged mature by the D L/S. Of those judged immature by RST, 92 per cent were also judged immature by the D L/S. The over-all rate of concordance between the RST and the D L/S was 68 per cent. The RST and the D L/S correlate linearly in a highly significant fashion (r = 0.71, p < 0.001). The results of using two indices of fetal maturity rather than one are expressed graphically in Figs. 4 and 5. Fig. 4 indicates that all of the fetuses that were mature by both the RST and the P L/S were clinically mature by pulmonary criteria. One fetus that was mature by the P L/S but immature by the RST developed mild RDS. Fig. 5 indicates that all fetuses that were mature by both the RST and the D L/S were clinically mature as well. One fetus that was mature by the RST, but immature by the D L/S, developed mild RDS. Infants who developed RDS. Table VII lists all the cases of RDS during our study interval. A total of 26 infants with RDS were delivered to 24 mothers. There were 13 infants with mild RDS, all of whom survived; the two infants with moderate RDS also survived; however, only four of the 11 infants with severe RDS survived. In our study, the small infant or the gestationally immature infant did not have a monopoly on RDS. The range of birth weights was 740 to 3,940 grams and the gestational age ranged from 25 weeks to 40 weeks. The infants who died of RDS had a mean birth weight of 2,100 grams and a mean gestational length of 34.6 weeks. All of the infants who developed RDS were the product of a high-risk pregnancy. Nearly two thirds of the 26 infants were delivered by cesarean

--

336

section. There were only three cases of uncomplicated vaginal delivery in the series and these three infants developed only mild RDS. Fetal distress, based on Hon’s24 criteria with electronic fetal monitoring, frequently was associated with mild RDS. Mature P L/S’s were associated with RDS in four instances (cases 3, 5, 6, and 10). Three of these infants developed severe RDS and two died. Case 19 represents the only infant in the study who had a mature RST before 34 weeks’ gestation; despite this, he developed mild RDS. Comment The ability to predict accurately the maturity of the fetal lung can help to determine the optimum time for delivery, since neonates with immature lungs may develop RDS. Maturation of the fetal lung appears to be associated with rising levels of surface-active lipid (SAL) in the amniotic fluid. A major component of the SAL is lecithin. The assay of Gluck and Borer and their colleagues’, rg In expresses a ratio of lecithin to sphingomyelin in the amniotic fluid. In our hands, this test has taken 1 to 3 hours to perform. The results of the P L/S have, in general, been reliable and reproducible. However, 6 per cent of the mature P L/S’s have been associated with clinical RDS and two neonatal deaths. The test also yields a 60 per cent level of false negatives in infants delivered within 72 hours of amniocentesis when interpreted as immature. In our series, a modification of their method, our D L/S, resulted in no RDS when a mature result was obtained. However, this test also produces a high level of false negative results (68 per cent of those infants classified as immature did not

Volume Number

121 3

Evaluation

Planimetric

L/S

Densltometrlc

Ratio -

o Delivered l

Delwered

72 hrs., no RDS

wthin

of

r

0

L/S

ratio

and

surfactant

test

329

Ratio 0 DelIvered

4, 7

at > 72 hrs., no RDS

L/S

l

DelIvered

Nithln

7: nr5, 10 l?DS

Cm0

at > 72 hrs, no RDS

A RDS 6.00 0 . .

5.00

:

Mature

00

4.00

*-

2 oc c-

.-.-..-..-2,oo Intermediate

1.00 Immature I 0

I

Immature

I

I

Intermediate

0

Mature

RST

L

-.-

Immature

..-L-L-Llnlermediale

MOlUi?

RST

Fig. 4. The P L/S ratio as a function of the RST. Fiftyfive cases are compared. The mean value of the P L/S ratio for each category of the RST is indicated.

Fig. 5. The D L/S ratio as a function of the RST. The comparison is from 336 cases. The mean value of the D L/S ratio for each category of the RST is indicated.

develop RDS when delivered within 72 hours of amniocentesis. With the use of the modification of the L/S ratio previously discussed, the mature limit of the D L/S may be lowered from 2.00: 1 .OO to 1.80 : 1 .OO without affecting the reliability of the prediction. However, it is critical that if any variation of Gluck’s technique is used, each laboratory must develop its own standard procedure and set its own limits for the L/S ratio. The RST of Clements and associatesg” apparently depends upon the absolute amount of SAL present in the amniotic fluid. The bubbles form only when the SAL reaches a certain concentration in the fluid; with higher concentration, they persist as a complete ring for 15 minutes. It is extremely important in performing the RST to adhere closely to Clements’ criteria, such as not moving the tube once the bubble ring has formed. In the 110 cases in our study in which delivery occurred within 72 hours of amniocentesis, when the RST suggested maturity, no RDS developed

in 109 cases (99 per cent). One sample did give a false indication of maturity by the RST (Table VII, case 19). This amniotic fluid was, however, contaminated with blood (hematocrit > 1.0). The 31 per cent false-negative rate in the RST immature category is much less than for either the P L/S (60 per cent) or the D L/S (68 per cent) ; thus, an infant with an immature RST was seen to be far more likely to develop RDS than an infant with an immature L/S ratio. This is further supported by the more significant correlation between the RST and the clinical status of the infant (p < 0.001) than between the P L/S and the clinical status of the infant (p < 0.05) or the D L/S and the clinical status of the infant (p < 0.005) . Thibeault and Hobel*” have similarly found that the incidence of RDS is significantly related to the RST (also at p < 0.001). In our experience, therefore, the more reliable test for fetal pulmonary maturity and immaturity is the RST. This is in contrast to the finding of Wagstaff and Bromham,?O who considered that

330

Keniston

Table VII.

et 01

Infants

with

KI>S

--.__

-I_-..---.

Case NO.

L/S ratio*

RSTf

Gestation (ulk.)

Days to delir’ery

Birth wt. (Cm.)

Mode

of

Pregnancy complication

1

1.49

--

34

2

1.89

-

37

3

4.5

-

37

3, I 70 Juvenile

4

1.33

-

35

1,620

5

2.40 2.00

-

37 34

2 25

6

2.45 1.68 0.84

-

36 35 31

7

1.92

-

36

8

1.85 1.37

-

9

1 .04

10

delivery

APgflr score I min./ 5 min.

Clinical

-.

course

1.820

Isoimmunization

Cesarean section

l/l

Erythroblastosis, died, HMD

2,900

Class

Cesarean section

8/5

Severe

diabetes

Cesarean section

3/7

Pneumothorax,

Placenta

previa

Cesarean section

6/8

Severe

2,660

Maternal

fever

Cesarean section

l/O

Severe RDS, hypoplastic

2 9 35

3,140

Placenta

previa

Cesarean section

8/6

Severe RDS, died, HMD fibrin clots at autopsy

0

2,460

Abrupti”

placentae

Cesarean section

l/4

Mild

RDS

35 35

3,660

Fetal

distress

Cesarean section

4/7

Mild

RDS

1

35

3,540

Class

C diabetes

Vaginal

8/9

Mild

RDS

2.16

1

36

2,810

Breech

Cesarean section

8/U

2 vessel cord,

mild

11

-

1

40

10

2,110

Fetal

distress

Cesarean section

l/O

Anencephalic, died, lungs autopsy

poor respirations, immature at

12

0.91

1

33

7

1,546

Fetal

distress

Cesarean section

6/7

Moderate

13

0.7 0.5

-

34 32

1 3

2,120

Premature rupture membranes

Vaginal

9/9

Mild

14

1.79 1.20 1.69 1.49 1.27

-~ -

0 0 0 2 15

2,240 2,200 1,970

Triplets

Vaginal (B and C breech )

5/5 7/9 5/6

1 1

34 34 34 34 32

Triplet A, mild RDS Triplet B, mild RDS Triplet C, mild RDS Sample from only one amniotic sac (which one is not known)

2 2

36 35

1 8

2,630

Isoimmunization

Cesarean section

l/2

Erythroblastosis, died, HMD

severe

33

16

2,690

fsoimmunization

Cesarean section

7/9

Erythroblastosis,

moderate

Cesarean section

l/6

Mild

Vaginal (forceps)

6/8

Hypospadias,

Vaginal (forceps)

9/9

Mild

Cesarean section

5/7

Severe

3

C diabetes

1.5 1.3

16

0.8

17

1.15

1

34

0

1,980

Premature rupture membranes

18

0.94

1

37

5

3,301)

Class

19

1.17

3

33

1

1,550

Premature rupture membranes

20

0.93

1

32

1

1,550

Severe ture

*Cases 1 to 10, P L/S; cases tl, Immature; 2, intermediate;

12 to 24, D L/S. 3, mature.

F.)

presentation

15

-

(107”

of

of

C diabetes

of

pre-eclampsia, prema_ . rupture ot membranes

severe RDS, at autopsy

RDS severe

RDS,

died,

RDS

no autopsy

died, lungs at autopsy plus

RDS

RDS

RDS

RDS

RDS

RDS

mild

RDS

RDS, RDS

Volume Number

121 3

Evaluation

of L/S ratio and surfactant

test

331

Table VII-Cont’d APw Case No.

L/S ratio*1

RSTf

Gestation (wk.)

Days to delivery

Birth wt. (Gm.)

21

2

32

1

1,930

22

1

25

0

740

1 1 1 1 1

35 35 34 33 33

0 2 7 11 15

3,300

23

1.77 1.23 1.04 0.85 0.64

score Pregnancy complication

Premature rupture membranes Premature

Mode of delivery

of

labor

Class C diabetes

24

1.53 1 38 0 3,940 Class C diabetes (juvenile) 0.81 1 38 3 1 37 6 0.65 1 37 10 1.0 1 36 13 0.57 1 34 25 *Cases 1 to 10, P L/S; cases 12 to 24, D L/S. tl, Immature; 2, intermediate; 3, mature.

“the test is valuable only as a screening procedure for determining fetal lung maturity.” Boehm and co-workers,‘l Roux and associates,” and Parkinson and Harvey,” however, have all had experiences similar to ours and reported the RST to be highly reliable, particularly when mature (“positive”) or immature (“negative” ). In addition to its greater reliability, the RST is far easier to perform than the L/S ratio. It requires only material and equipment that might easily be carried to the bedside. If the amniotic fluid sample is uncontaminated with blood or meconium, and the bubbles persist at the 1:2 dilution for 15 minutes, the RST may give a nearly 100 per cent assurance of fetal maturity within 30 minutes. In cases with blood or meconium contamination, the D L/S may be used to give a further estimate of fetal maturity. In this series the L/S ratio gave no false indications of maturity on the basis of sample contamination, either with blood at a hematocrit greater than 1.0 and/or with meconium staining. (No sample giving false indications of maturity by the P L/S was contaminated.) As noted above, one RST analysis did give a false indication of maturity. The D L/S of this sample was 1.17. Thus, in our series, the analysis of the contaminated sample consistently proved clinically

I min./ 5 min.

Clinical Severe RDS

course

Vaginal (forceps )

2/2

Vaginal (double footling breech)

l/O

No respiratory effort, died, lungs immature at autopsy

Vaginal

9/9

Mild

RDS

Cesarean section

5/7

Mild

RDS

useful, particularly if both RST and D L/S were employed. However, it should be noted that very heavily contaminated samples may give false indications of maturity, as has been shown by Wagstaff and associates.‘” Using either the RST and the P L/S or the RST and the D L/S combined, we had no cases of RDS when both tests indicated fetal maturity. In infants delivered within 72 hours, and predicted to be mature by both tests, or mature by the RST and intermediate by the D L/S, or intermediate by the RST and mature by the D L/S, we had no cases of RDS. Therefore, when the RST is intermediate, the D L/S can be used to differentiate the possibly immature fetus (D L/S < 1.50: 1 .OO; 22 per cent incidence of RDS) f rom the likely mature fetus (D L/S 2 1.80; RDS in none).

The authors extend their appreciation to Thomas Porter, M.D., and Ms. Bonnie Weiner for much of the early work in performing the P L/S and Ms. Shirley Jensen, M. T. (A.S.C.P.), for performing the D L/S and RST. The assistance of S. Gorham Babson, M.D., Director of the University of Oregon Medical School’s Neonatal Intensive-Care Unit and Nurseries, is also gratefully acknourledged.

332

Keniston

et al.

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