Point of Care Testing Perils and Pitfalls

LAB UPDATE Feb. 15, 2006 Dr. Beverly Dickson Heparin Induced Thrombocytopenia An antibody mediated adverse effect of heparin that is important because of its strong association with venous and arterial thrombosis. Heparin exposure may be any preparation (LMWH) by any route (flushes), and with any dose. HEPARIN INDUCED THROMBOCYTOPENIA Clinicopathological Syndrome Clinical: Serologic: Thrombocytopenia with or without thrombosis High titer platelet activating HIT antibodies by sensitive antigen and/or activation assays Clinical Features of HIT Timing of thrombocytopenia Typical onset: between days 5 and 10 after starting heparin Rapid onset: < 1 day following resumption of heparin (usually in a patient recently exposed to heparin, who therefore has residual circulating HIT antibodies) Severity of thrombocytopenia Platelet count nadir: < 20,000/µL in 10% of patients; < 150,000/µL in 85% of patients Thrombosis is common > 50% develop new thrombosis Venous thrombosis: deep venous thrombosis > pulmonary embolism > warfarininduced venous limb gangrene > adrenal hemorrhagic necrosis* > cerebral sinus thrombosis Arterial thrombosis: limb artery thrombosis > stroke syndrome > myocardial infarction > mesenteric artery thrombosis Absence of petechiae (even with platelets < 20,000/µL) Skin lesions at heparin injection sites Severity ranges from erythematous plaques to skin necrosis Acute systemic reactions following intravenous bolus heparin Acute inflammatory or cardiorespiratory signs and symptoms associated with abrupt platelet count fall Consultative Hemostasis and Thrombosis, 2002, pg358. Consultative Hemostasis and Thrombosis 2002, pg 359 DRUG INDUCED THROMBOCYTOPENIA MECHANISMS Heparin-induced Thrombocytopenia Frequency Onset after beginning treatment Platelet count Sequelae Approx. 1/100 5-14 days Quinine- or Sulfainduced Thrombocytopenia Approx. 1/10,000 ≥ 7 days 20 - 150x109/L* Thrombosis < 20x109/L Bleeding Drug-dependent increase in platelet-associated IgG Laboratory testing Heparin-dependent platelet activation; Immunoassay using patient (heparin/PF4 antigen) serum *Some patients have a fall in platelet count but platelet count remains > 150x10 9/L Simposio Internacional CLAHT PERU 2004 HIT ASSAYS Two Major Classes Functional activation assays (Serotonin Release Assay): Infer presence of HIT antibodies based on heparin-dependent, platelet activating properties (washed platelets) Antigen Assays (Heparin-PF4 ELISA): Detect HIT antibodies based upon their reactivity with platelet factor 4 (PF4) complexed to heparin or other polyanions (ELISA) Specificity, % HIT Diagnostic Assay Sensitivity, % Early Platelet Fall Late Platelet Fall Platelet SRA 90-98 * >95 80-97 ‡ Heparin-induced platelet aggregation assay Platelet aggregation test using citrated platelet-rich plasma PF4/heparin EIA Combination of sensitive platelet activation and PF4-dependent antigen assay 90-98 * >95 ‡ 80-97 ‡ 35-85 >90 * 100 * 90◊ >95 >95  82◊ 50-93 80-97   ”Early” refers to a fall in the platelet count that begins within the first 4 d of starting heparin; “late” refers to a fall that begins on day 5 or later. The specificity varies because late thrombocytopenia due to a reason other than HIT may nevertheless show a false-positive HIT antibody result because of subclinical HIT antibody seroconversion. *Sensitivity defined in relation to those patients in prospective studies who had a positive test result when the platelet count fell by ≥50% after ≥5 days of heparin therapy, and in whom the available clinical information (particularly, evidence for alternative explanations for thrombocytopenia and the effect of stopping or continuing heparin) supported the diagnosis of HIT. However, about 30-40% of samples (app. 2% overall) give a repeated “indeterminate” result, and the activation assay is nondiagnostic. ‡ Assumes that the heparin-induced platelet aggregation assay test and SRA have similar sensitivity and specificity profiles; other platelet activation end points that may also give acceptable results using washed platelets include detection of platelet-derived micro particles by flow cytometry. ◊ Assumes that a 90% specificity in early thrombocytopenia attributable to non-HIT disorders (eg, nonspecific platelet activation related to acute inflammatory proteins) declines to an 82% specificity in late thrombocytopenia that may be attributable to subclinical HIT antibody seroconversion.  Clinicopathologic definition assumes that at least one sensitive test result must be positive for diagnosis of HIT; specificity of the activation assay is indicated. Localization of thromboembolic complications associated with HIT Thromb Haemost 2005:94:132-5. Type of TEC Arterial Limb artery Thrombotic stroke Number of TECs (%) 126 (29.2%) 71 (16.4%) 26 (6.0%) Aortic Thrombosis Myocardial infarction Other Venous Proximal DVT 16 (3.7%) 10 (2.3%) 3 (0.7%) 306 (70.8%) 114 (26.4%) Pulmonary embolism Distal DVT Cerebral vein (sinus) thrombosis Other 103 (23.8%) 78 (18.1%) 7 (1.6%) 4 (0.9%) Simposio Internacional CLAHT PERU 2004 Clinical Assessment: Inclusion Criteria History of heparin exposure Thrombocytopenia during and after heparin exposure (<150,000) Drop in platelet count (<50%) rather than absolute thrombocytopenia Smaller drop in platelets (especially skin necrosis) Early-onset of thrombocytopenia with heparin re-exposure caused by circulating antibodies Platelet count may rarely be normal when patient presents with thrombosis (delayed-onset HIT) Thrombocytopenia recovers after heparin withdrawal Median time to platelet count recovery after heparin withdrawal is 4 days Platelet Monitoring for HIT ACCP Consensus Conference, 2004 HIT risk >0.1% - platelet count monitoring Patients recently treated with heparin starting UFH Platelet count baseline within 24 hours Acute systemic reaction post UFH bolus Immediate platelet count Patients receiving therapeutic dose UFH Every other day platelet count until day 14 or UFH stopped, whichever is first Postoperative patients, UFH prophylaxis (HIT risk >1%) Every other day platelet count until 14 days or UFH stopped. HIT Patient Risk Groups 1-5% risk (highest) Post-op vascular, ortho, cardiac patients receiving UFH for 1-2 weeks 0.1-1% risk (rare to infrequent HIT) Medical and obstetric patients receiving prophylactic doses of UFH Post-op patients receiving LMWH Post-op/critical care patients with UFH flushes Medical patients receiving LMWH after one or several preceding doses of UFH HIT A negative laboratory test for HIT antibodies should never be used as the sole criterion for restarting heparin therapy. HIT CONCLUSIONS Routine platelet monitoring rather than HIT Antibody studies is most useful to identify patients who are at risk for thrombosis Although functional and antigen assays are sensitive in detecting HIT antibodies, neither is completely specific for HIT syndrome The diagnostic interpretation of these laboratory tests must be made in the context of clinical pretest probability of HIT Clindamycin Disk Inductive Test for Staphylococcus spp. “D Test” • Routinely performed on Staphylococcus that test resistant to erythromycin but are susceptible to clindamycin • Many MRSA that cause community acquired infections have msrA gene D Test Resistance to macrolides (e.g. erythromycin) can occur by two different mechanisms with the resulting phenotypes noted below: Mechanism Determinant (gene) msrA Erythromycin Clindamycin Efflux R S Ribosome alteration erm erm R R S* R (constitutive) msrA=macrolide streptogramin (type B) resistance Erm=erythromycin ribosome methylase; encodes enzymes that confer inducible (MLS Bi) or constitutive (MLSBc) resistance to MLS agents via methylation of the 23S rRNA *requires induction to demonstrate resistance MLS=macrolide lincosamide (e.g. clindamycin) streptogramin (type B) Inducible Clindamycin Resistance (erm-mediated) Helicobacter pylori Tests at PHD CLO-Test HpSA Rapid urease Stool antigen test Sensitivity Specificity Invasive Non-invasive FDA Detects active approved test infection for cure CLO HpSA 80-98% 93-100% Yes Yes 92-97% 80-95% 90-95% 80-95% Yes No Yes No Serology Helicobacter pylori AGG/AGA Graded Recommendations 2005 For patients ≤ 55 years without alarm features, the clinician may use either “test and treat” for H. pylori or acid suppression therapy. (A) Point of Care PT Blood Gas (TIS) 6+ I-STAT ACT i calcium creatinine Additional POC instruments: Hemocue, Accu-Chek, Clinitek, DCA 2000 (A1c) Point of Care Limitations Method dependent Technique dependent Interferences: known and unknown Drugs Metabolic Other Chemical Pathology Are your hands clean enough for point-of-care electrolyte analysis? Hugh S. Lam*, Michael H.M. Chan, Pak C. Ng*, William Wong*, Robert C.K.Cheung, Alan K.W. So*, Tai F. Fok* and Christopher W.K. Lam Departments of *Paediatrics and Chemical Pathology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong ______________________________________________________________________ Pathology (August 2005) 37(4) pp.299-304 Case #1 POC/INR The patient is a 75 year old female with history of frequent TIA, recently placed on coumadin Upon ambulatory clinic visit her I-STAT INR was > 8.0 but Core Lab INR was 4.1 What might cause this discrepancy? → technique → interference → reagents → instrument malfunction POC/INR Interferences Lupus anticoagulant/anticardiolipin Ab LMWH Direct thrombin inhibitors Daptomycin Case #2 ED/I-STAT +6 I-STAT Core Lab A middle aged male presented to the ED post grand mal seizure. Patient had a history of hyponatremic episodes secondary to anti-convulsant drug therapy. Na 132 140 K Cl BUN AG Glu 3.7 103 13 N/A 159 4.0 100 10 31 158 Hgb 17 14 Accu-Chek Inform Meter Accu-Chek Comfort Curve Strip A whole blood glucose test strip that delivers plasma-like test results. Case #3 ICU/Accu-Chek Inform Meter The patient is a 75 year old male who was transferred from Lake Pointe in septic shock s/p hernia repair. The patient developed renal failure, liver failure with coagulopathy and respiratory failure. Multiple POC glucoses performed, multiple results discrepant with laboratory. Date Jan 9 Time* 0754 Core Lab 109 AccuChek 140 Jan 9 Jan 8 0550 1415 88 63 129 99 Jan 8 Jan 8 Jan 8 1055 0630 0350 59 36 38 100 75 74 * Results charted within minutes of each other Case #3 Metabolic Status Alk Phos ALT 374 U/L 407 U/L AST Total Bili Lipase CK 2697 U/L 5.4 mg/dL 2347 U/L 863 U/L Creatinine TnI Calcium i Calcium Anion gap 4.7 mg/dL 0.5 ng/mL 5.5 mg/dL 0.71 mmol/L 24 PT INR Lactic Acid 32.8 sec 3.0 10.7 mmol/L Accu-Chek Comfort Curve Strip Known Interferences Galactose Maltose Xylose Bilirubin (> 20 mg/dL) Lipemia (> 5000 mg/dL) Acetaminophen (> 8 mg/dL) Uric acid Low Hct (< 20%) High Hct > 65% @ ≤ 200 mg/dL > 55% @ > 200 mg/dL Mannitol Icodextrin 60 mg/dL glucose concentration 500 mg/dL glucose concentration Clinica Chimica Acta 356 (2005) 178-183. CAPILLARY BLOOD GLUCOMETER VS REFERENCE STANDARD Crit Care Med 2005 Vol 33, no.12. ARTERIAL BLOOD GLUCOMETER AND REFERENCE STANDARD Crit Care Med 2005 Vol 33, No 12 POC Glucose meters originally designed to test glucose in diabetics with normal hematocrit ICU patients may suffer from multiple metabolic +/or hematologic derangements ICU patients may be treated with multiple drugs Results which do not seem realistic in view of the clinical assessment should be repeated in the Core Laboratory. Urine Drug Screen Immunoassay Does it detect oxycodone? Basic urine drug immunoassay testing for opiates tests primarily for morphine (heroin and codeine metabolized to morphine). These tests do not generally detect low to moderate oxycodone use. STAT Quantitative Serum Toxicology Assays Required to Support the ED acetaminophen lithium salicylate theophylline valproic acid co-oximetry digoxin phenobarbital iron transferrin ETOH MEOH ethylene glycol Oxycodone To detect compliance, abuse or toxicity best detected by specific assay detection levels < 100ug/L necessary Urine Drug Screen Immunoassay Drug PCP Opiates Amphetamines X-reaction Dextromethorphan, diphenhydramine, sertraline Quinolone antibiotics Detects all types of sympathomimetic amines (including those in OTC diet suppressants and cold medications)