Annals of African Medicine, Vol. 5, No. 1, 2006, pp. 38-41

Relationship between Serum Heat-Stable Alkaline Phosphatase Activity and Blood Pressure in Patients with Pre-Eclampsia and Eclampsia

¹I. S. Aliyu, ¹H. S.  Isah and ²O. A. Afonja

¹Department of Chemical Pathology, AhmaduBelloUniversity Teaching Hospital, Zaria and ²Department of Clinical Pathology, LagosUniversity Teaching Hospital, Lagos, Nigeria

Reprint requests to: Dr I. S. Aliyu, Department of Chemical Pathology, A. B.U. Teaching Hospital, Zaria, Nigeria. E-mail: iasambo@yahoo.com

 Code Number: am06009

Abstract

Background: The objective of this study was to explore the relationship, if any, between theserum heat-stable alkaline phosphatase (HS-ALP) activity and the blood pressure (BP) of patients with pre-eclampsia and eclampsia. 
Method: The activity of HS-ALP was measured using the 4 – nitrophenyl phosphate (4 – NPP) method after incubation at a high temperature of 65 ⁰C for exactly 30 minutes in one hundred normal pregnant women and in another one hundred with pre-eclampsia/eclampsia. The normal pregnant women were used as controls. The blood pressure (BP), systolic as well as diastolic was measured in each of the studied patient using desktop mercury sphygmomanometer.
Results:In the patients with pre-eclampsia/eclampsia, it was found that the higher the systolic and diastolic BP, the higher is the activity of the HS-ALP.
Conclusion: It can be concluded that the HS-ALP activity in patients with pre-eclampsia/eclampsia is positively related to the severity of the hypertension and therefore this could help in detecting early complication. 

Key words:Heat-stable alkaline phosphatase, eclampsia, blood pressure

Résumé

Fond : L'objectif de cette étude était d'explorer le rapport, s’il y en a, entre l'activité thermostable de la phosphatase alkaline de sérum (TS-PAL) et la tension artérielle (TA) des malades avec la pré-éclampsie et l'éclampsie.
Méthode : L'activité de TS-PAL a été mesurée en utilisant La méthode de 4-phosphate nitrophénylique (4-PPN) après l’incubation à température élevée de 65 0C pendant exactement 30 minutes dans cent femmes enceintes normales et dans encore cent avec l' pre-éclampsie/éclampsie. Les femmes normalement enceintes ont été employées comme commandes. La tension artérielle (TA), systolique aussi bien que diastolique a été mesurée dans chaque malade étudié en employant  le sphygmomanomètre  mecure de bureau intelligent.
Résultats: Dans les malades avec la pre-éclampsie et éclampsie, on a constaté que plus la TA  systolique et diastolique est haute, plus est haute l'activité du TS-PAL .
Conclusion: On peut conclure que l'activité de TS-PAL dans les patients avec la pre-éclampsie/éclampsie est franchement lié à la sévérité de l'hypertension et donc ceci pourrait aider pour détecter la  toute promiére complication.

Mots clés: Phosphatase alcaline thermostable, éclampsie, tension artérielle

Introduction

Serum alkaline phosphatase measurements are  important in investigating hepatobiliary diseases associated with biliary obstruction¹, bone diseases associated with increased osteoblastic activity² and, to some extent placental insufficiency.³

Several tests have been extensively used to assess foeto-placenta function and development during pregnancy. The placenta is a rich source of enzymes, most of which are of course common to other tissues, but heat-stable alkaline phosphatase, cystyl-aminopeptidase (oxytocinase) and diamine oxidase (histaminase) are relatively specific to this organ.⁴

Serum alkaline phosphatase is elevated in pregnancy, especially during the second and third trimesters.  The activity of this enzyme begins to rise at the fourth (4^th) month of gestation.  This elevation is a reflection of placental fraction entering the maternal blood, since the human trophoblastic cells are rich in alkaline phosphatase. Therefore, measurement of serum alkaline phosphatase of placental origin is of particular interest in the investigation of placental insufficiency.⁵

The increase in the serum alkaline phosphatase during the second half of pregnancy was recognised and confirmed by many investigators. ^{6 -12} It was also further confirmed that the alkaline phosphatase that originated in the placenta was heat-stable. ^{13 - 17}The increase, which becomes apparent during the second trimester, continues throughout the third trimester reaching a peak at term, but this disappears within a few weeks after delivery.

Abnormal elevation in the activity of serum heat-stable alkaline phosphatase has been reported in patients with pre-eclampsia few weeks before clinical signs become apparent.^{18 -22} On the other hand, abnormally low values have been observed in mothers whose pregnancies were at risk owing to hypertension or diabetes.²³

Patients and Methods

 Patients

One hundred pregnant women in the third trimester who presented to the antenatal clinic of Ahmadu Bello University teaching hospital, Zaria with pre-eclampsia/eclampsia or who were admitted into the maternity or labour wards with pre-eclampsia or eclampsia were selected for the study.  The usual diagnostic criteria of oedema, proteinuria and systemic hypertension were used in selecting the pre-eclamptic patients while those with eclampsia had grandmal intrapartum convulsions. The ages of this    group of patients ranged between 16-39 years with a mean of 26.6 years.

Another one hundred (100) women with normal pregnancy at different stages of the third trimester attending the same clinic were used as control. The age of this group of subjects ranged from 17-45 years with a mean of 26.7 years. In all selected subjects, gestational age was determined from the date of last menstrual period (LMP) and clinical measurement of the fundal height and ultrasonographic estimation were also conducted.

The selected patients with pre-eclampsia/eclampsia were further classified into mild pre-eclampsia (were the BP is above 130/90 mmHg but less than 160/100 mmHg with associated oedema and proteinuria), severe pre-eclampsia (when the BP is more than 160/100 mmHg with associated oedema and proteinuria) and eclampsia (when there was a convulsion in addition to the above features).

Approval for the study was obtained from the Ethical Committee of Faculty of Medicine, AhmaduBelloUniversity, Zaria and informed consent was obtained from all patients.

 Methods

Ten millilitres of venous blood specimen was collected by venepuncture from the ante-cubital vein from each patient. The skin over the site of collection was swabbed with methylated spirit and allowed to dry.  A tourniquet was then applied 10 cm above the ante-cubital fossa to make the veins distended with blood.  Sterile 21G size hypodermic disposable needles were used to draw 10 ml of blood from patients and controls. The blood was then transferred into plain specimen bottles containing no anticoagulant.  After 30 – 45 minutes when the sample would have clotted, serum separation was carried out by centrifugation at 4000 rpm for 10 minutes. 

The activity of serum heat-stable ALP was then measured.  Sample analysis was carried out for every specimen on the same day of collection. The 4 – nitrophenyl phosphate (4 – NPP) method, which is in use in the Ahmadu Bello University Teaching Hospital (ABUTH) Chemical Pathology laboratory, was used for the analysis of samples in this study. The ALP hydrolyses 4 – nitrophenyl phosphate (4 – NPP) at a pH of 10.3 and temperature of 37 ⁰C to liberate 4 – nitrophenol (yellow complex). The total serum ALP was obtained by reading the absorbance of the 4 – nitrophenol at 510 nm using the spectrophotometer. The serum heat stable alkaline phosphatase activity was determined in the same way except for incubation at 65 ⁰C for exactly 30 minutes to inhibit alkaline phosphatase activity derived from sources other than the placenta, which remain stable at this temperature.

The within-run and between-day precision profiles were assessed using pooled sera. The within –run coefficients of variations (CV) for low, medium and high ALP activities were 3.67%, 0.77% and 0.35% respectively, while the corresponding values for the between-run were 6.02%, 1.27% and 0.95%, thus, the method was found to be precise.

Quality control sera provided by BIOTEC Laboratories Ltd, with low, normal and high levels of ALP activity were analysed with each batch of assays to ensure their validity.

Distributions of ALP activities were found to be skewed, hence non-parametric analysis using percentiles were used in measuring the variation. 

Results

Table 1 shows the clinical and laboratory characteristics of the controls and the study patients with pre-eclampsia/eclampsia. There was no statistically difference between the maternal and gestational ages among the studied patients and the control group (P > 0.05) while the mean blood pressure in the controls was significantly lower (P < 0.001) than in the patients with pre-eclampsia/eclampsia.

There was a higher HS-ALP activity in patients than in controls. This difference in HS-ALP activity between the normal pregnant women and that of the patients with pre-eclampsia/eclampsia is statistically significant (p<0.001).

Table 2 showed a normal systolic blood pressure (SBP) in the controls with no significant difference in the HS-ALP activity where as in patients with pre-eclampsia/eclampsia as the SBP increases, the HS-ALP activity increases, hence the higher the SBP, the greater is the HS-ALP activity. This relationship is statistically significant (p < 0.05).

Table 3 shows the relationship between DBP and HS-ALP activity for both the controls and patients with pre-eclampsia/eclampsia. There is gradual rise in HS-ALP activity with increase in DBP. The observation is also statistically significant   (p < 0.05).

Table 1: Clinical and Laboratory characteristics of control subjects and patients with pre-eclampsia/eclampsia

Characteristics	Mean for controls (range)	Mean for study group (range)	p value
Age (Years)	26.7   (17 – 45)	26.6 (16 – 39)	>0.05
Gestational age (weeks)	34.5 (28 – 42)	35 (28 – 42)	>0.05
Systolic blood pressure (mmHg)	110.7 (90 – 130)	158.1 (130 – 200)	<0.001
Diastolic blood pressure (mmHg)	70.4 (50 – 80)	107.9 (90 – 150)	<0.001
Serum HSALP (IU/L)	74.5 (14 – 166)	144.6 (17 – 352)	<0.001

HSALP: serum heat-stable alkaline phosphatase

Table 2: Systolic blood pressure and heat-stable alkaline phosphatase activity in controls and in patients with pre-eclampsia/eclampsia

 

Control group			Study group
Systolic blood pressure (mmHg)	n	HSALP  (IU/L)	Systolic blood pressure (mmHg)	n	HSALP  (IU/L)
90 – 100	26	72.5	131 – 140	17	119.9
101 – 110	37	74.9	141 – 160	50	139
111 – 130	37	77.4	161 – 200	33	163.9
Total	100		100

HSALP: serum heat-stable alkaline phosphatase; Results of HSALP activities are presented as mean

Table 3: Diastolic blood pressure and heat-stable alkaline phosphatase activity in controls and in patients with pre-eclampsia/eclampsia

Control group			Study group
Diastolic blood pressure (mmHg)	n	HSALP  (IU/L)	Diastolic blood pressure (mmHg)	n	HSALP  (IU/L)
50 – 65	21	68.7	91 – 105	52	128
66 – 90	79	76.1	106 – 120	42	160.8
			121 – 150	6	175.5
Total	100		100

HSALP: serum heat-stable alkaline phosphatase; Results of HSALP activities are presented as mean