Lipase

Physiology

Lipases hydrolyze triglycerides. There are several forms of lipase found in various tissues: pancreatic lipase, colipase, hepatic lipase, gastrointestinal lipases, renal lipase and lipoprotein lipase. Lipoprotein lipase is produced by vascular endothelium and is essential in triglyceride metabolism. Based on sequential decreases in lipase activity in ill horses, lipase (as measured by the DGGR assay, see below) appears to have a half life of approximately 11 ± 7 hours in this species (Johnson et al 2019). In horse tissues, the highest lipase activity is seen in the pancreas, with small amounts in salivary gland and jejunum. By virtue of tissue mass, lipase activity is also seen in small amounts in liver, muscle, and adipose tissue (Johnson et al 2019).

Methodology

Assay methods

Lipase activity: These are based on hydrolysis of an olive oil emulsion into constituent fatty acids.

• Titrometric: This estimates the quantity of sodium hydroxide needed to neutralize fatty acids produced by lipase activity.
• Nephelometry: Triolein (long chain fatty acids) is degraded to diglycerides. The degradation of triolein results in decreased turbidity. Hemolysis and icterus can result in falsely low activity, whereas lipid can increase activity.
• Colorimetric: Measures the rate of dye formation yielded from the breakdown of a substrate of long chain fatty acids (such as 1,2-o-dilauryl-rac-glycero-3-glutaric acid or 1,2-diglyceride), and is the current procedure used at Cornell. 

Pancreatic lipase (PL) immunoreactivity: These are based on immunologic detection of canine and feline pancreatic lipase and are specific for this enzyme. The assay does not react with other lipases, unlike activity assays, which can measure activity of more than just pancreatic lipase. There are various available immunoassays used for quantification or semi-quantification of pancreatic lipase immunoreactivity, with the most commonly used quantitative assay being the Spec cPL ELISA (review by Cridge et al 2021). Semi-quantitative assays were designed to be used at point-of-care and with a rapid turnaround. Correlation of results in 50 dogs between various point-of-care assays and the DGGR assay ranged from 0.86 to 0.96 (good to excellent correlation) (Cridge et al 2018). Similarly, there was good agreement between a point-of-care and ELISA-based fPL assay in 111 cats (70% of which had results inconsistent with pancreatitis) (Schnauβ et al 2019). The inter-assay co-efficient of variation of the Spec cPL assay was lower (<9%) than 2 point-of-care assays (range, 5-41%, with >20% being unacceptably high) (Cridge et al 2020). 

Procedure at Cornell University

• Activity assay: The assay uses a 1,2-o-dilauryl-rac-glycero-3-glutaric acid- (6’-methylresorufin) ester (DGGR) as a substrate for lipase activity. The DGGR ester is cleaved by the catalytic action of lipase, in an alkaline solution, to form 1,2-O-dilauryl-rac-glycerol and an unstable intermediate, glutaric acid-(6-methylresorufin) ester. This decomposes spontaneously in alkaline solution to form glutaric acid and methylresorufin. The rate of formation of methylresorufin, which is a red dye, is measured. Addition of a detergent and colipase is supposed to increase the specificity of the assay for pancreatic lipase, but the presence of a reaction in dogs with exocrine pancreatic insufficiency indicates that other non-pancreatic lipases react in this assay (Cridge et al 2021).
• Reaction type: Kinetic enzymatic colorimetric.
• Limits of quantification: 3-300 U/L per manufacturer. Higher results are reported out after dilution to get results within this range.
• Lower limit of detection: 2 U/L (mean ± 3 SD of a blank, per package insert). In-house studies of a blank solution yielded zero activity. Serial dilution of a sample of 256 U/L showed linearity to as low as 2 U/L.
• Precision: In-house studies shows an intra-assay coefficient of variation of 0.6-1.3% in samples from a kit from the manufacturer that is designed to measure precision. 

Sample considerations

Sample type

Serum or heparinized plasma, body cavity fluid samples. 

Stability

Stability in human samples is 1 week at room temperature or 4ºC and 1 year at -20ºC (per package insert). Lipase activity in canine serum with the DGGR assay did decrease with frozen storage after 20 days (Graca et al 2005).

Interferences

• Lipemia, hemolysis, and icterus: No interference is seen in human samples with a hemolytic index up to 1000 units, turbidity index of up to 2000 units (the turbidity index correlates weakly to triglyceride concentrations and lipemia) or an icteric index up to 60 units. Spiking of canine samples with hemoglobin to a hemolytic index of 1000 units or 20% intralipid® did not affect activities (Graca et al 2005).
• Drugs: Corticosteroids are reported to increase lipase activity with the 1,2 diglyceride substrate. In addition, administration of heparin to dogs and cats (to release lipoprotein or hepatic lipase) increases lipase activity using this method, supporting a lack of specificity of the DGGR substrate for pancreatic lipase. Lipase activity after administration of unfractionated heparin (50 IU/kg) peaked at 10 minutes (increased by a median of 5 and 10 U/L in dogs and cats, respectively, with individual animals having higher increases) and returned to baseline at 120 minutes. The clinical relevance of these findings is unclear (increases in dogs were not above established reference intervals for lipase) (Lim et al 2020).

Test interpretation

Measurement of lipase is most commonly performed to diagnose pancreatitis. Reported causes of increased lipase activity appear to be substrate dependent. In one study, the older 1,2 diglyceride-based assays appeared less sensitive than the newer DGGR-based assay (see below). 

Increased activity (hyperlipasemia)

• Pathophysiologic
  • Acute pancreatitis: Destruction of pancreatic acinar tissue results in the escape of pancreatic enzymes into the pancreas and peritoneal cavity. The enzymes enter the blood by way of lymphatics or capillaries with subsequent increase in serum or plasma activity. 
    • ELISA Spec cPL and fPL: These are considered the “gold standard” assays for diagnosis of pancreatitis, being sensitive and specific tests in dogs and cats, respectively. However, these assays still suffer from false positive and negative results (Xenoulis and Steiner 2012).
      • Spec cPL: Results >400 ug/L are compatible with pancreatitis, results between 200 and 400 ug/L are considered equivocal (retesting recommended) and results <200 ug/L are not compatible with pancreatitis (Cridge et al 2018, 2021). 
      • Spec fL: Cats have lower pancreatic lipase immunoreactivity versus dogs. Values > 5.4 ug/L are compatible with pancreatitis, between 3.6 and 5.4 ug/L are equivocal, and <0.7 ug/L, inconsistent with pancreatitis (Schnauβ et al 2019). Cats with chronic pancreatitis can frequently have normal concentrations of fPL.
    • DGGR assay: 
      • Dogs: Graca et al (2005) compared lipase activity results with the DGGR and 1,2 diglyceride assays in 30 dogs with vomiting and diarrhea, 50% of which had pancreatitis. Dogs with pancreatitis had to have 1 or more of anorexia, vomiting, diarrhea, depression and physical examination findings of 1 or more of dehydration, abdominal pain, icterus, or fever and ultrasonographic evidence of pancreatitis (which is not sensitive or specific). They found the DGGR assay had 93% sensitivity and 53% specificity versus the 1,2 diglyceride assay, which had lower sensitivity of 60% and possibly higher specificity of 73% (no confidence intervals provided). Both assays had similar area under the curve of 0.7 (reasonable performance). Another study by Kook et al (2014) compared this assay to the Spec cPL (ELISA-based assay for pancreatic lipase), which is considered the gold standard, in 142 dogs. The results showed good agreement with the Spec cPL (0.795 kappa coefficient at a lipase activity of >108 U/L and >200 ug/L spec cPL or suspicion of pancreatitis). Our established intervals on clinically healthy dogs (n=122) have a higher upper limit (228 U/L) than that published previously for dogs (Graca et al 2005 [n=56], Kook et al 2015, [n=75]).
      • Cats: A study by Oppliger et al (2013) showed good concordance with the Spec fPL in cats with pancreatitis (both clinically diagnosed and histologically confirmed cases with a kappa coefficient of 0.68, at a lipase activity of >26 U/L and >5.3 ug/L Spec fPL). The results of either assay (Spec fPL or DGGR-based lipase, using the above cut-offs) did not correlate well with ultrasonographic evidence of pancreatitis in a study of 161 cats with suspected pancreatitis (Oppliger et al 2014).  Our upper reference limit for cats (23 U/L; n=102) is similar to that published previously (Oppliger et al 2013, n=80).
      • Horses: The pancreas is the only relevant source of lipase, as measured by the DGGR assay. High lipase activity (>20 U/L) was seen in 40% of horses admitted to one veterinary institution in Europe and was interpreted as pancreatitis. Affected horses mostly were presented for gastrointestinal disease and colic, but mildly increased lipase activities were also seen in various other conditions and thoroughbreds in training. Increases in lipase activity only correlated to increases in amylase activity (correlation coefficient of 0.88) with marked increases in lipase (>100 U/L) (Johnson et al 2019). 
    • 1,2 diglyceride assay: Increases of at least 2 x normal were seen in dogs and cats with pancreatitis. In dogs, lipase activity increases within 24 hours and peaks (at a higher level than amylase) at 2-5 days. Increases of 3 times normal supported a diagnosis of pancreatitis more strongly than more moderate increases, which may be accounted for by non-pancreatic causes. Lipase activity may be normal in up to 28% of dogs with pancreatitis. In the cat, lipase activity is not consistently increased in pancreatitis. Using this assay, we have diagnosed pancreatitis in horses based on lipase activity in abdominal fluid > activity in serum (Ollivett et al 2012). The diagnostic utility of lipase activity in horses remains to be assessed. Lipase activity did not increase for up to 8 hours after eating in 100 clinically healthy dogs (Yi et al 2022).
  • Gastrointestinal disease:
    • DGGR assay: There is limited data available. In house studies on low numbers of animals suggests that lipase activity may be increased in dogs with gastrointestinal disease, but it is unclear if these dogs have concurrent mild pancreatitis or if the assay is picking up a non-pancreatic form of lipase.
    • 1,2 diglyceride assay: Peritonitis, gastritis, bowel obstruction, and visceral manipulation (laparotomy) may increase lipase activity by 2-3 x normal. As stated above, the mechanism for the increase is unclear.
  • Hepatic disease:
    • DGGR assay: No data available.
    • 1,2 diglyceride assay: Hepatic disease and neoplasia have been associated with increased activities. The changes with hepatic disease could reflect concurrent pancreatitis.
  • Azotemia:
    • DGGR assay: There is limited data available. In-house studies on low numbers of animals suggests that lipase activity is not always increased with severe azotemia in dogs. It can be increased in a few dogs with azotemia, but the mechanism is unclear (there could be concurrent mild pancreatitis).
    • 1,2 diglyceride assay: Increases of up to 4 x normal activity may be seen in patients with decreased GFR. In azotemic patients with lipase activity greater than 3-4 times normal, a diagnosis of pancreatitis should still be considered.
  • Hyperadrenocorticism (HAC): Dogs with HAC can have increased lipase activity with the DGGR assay (up to 874 U/L, n = 22 dogs, reference interval upper limit, 118 U/L) and 1,2-diglyceride assay (up to 1930 U/L, reference interval upper limit, 582 U/L), however pancreatitis was not confirmed in these dogs by measurement of canine pancreatic lipase (only ultrasonography was done) (Linari et al 2021).